unit tests: replace nose with pytest (#2502)

* Porting: substitute nose with ytest

This huge commit substitutes nose with pytest as a testing system. Things done here:

* deleted external/nose as it is no longer used
* moved mock resources in their own directory 'test/mock/'
* ported two tests (cmd/find, build_system) to pytest native syntax as an example
* build_environment, log: used monkeypatch instead of try/catch
* moved global mocking of fetch_cache to an auto-used fixture
* moved global mocking from test/__init__.py to conftest.py
* made `spack test` a wrapper around pytest
* run-unit-tests: avoid running python 2.6 tests under coverage to speed them up
* use `pytest --cov` instead of coverage run to cut down testing time

* mock/packages_test: moved mock yaml configuration to files instead of leaving it in the code as string literals

* concretize.py: ported tests to native pytest, reverted multiprocessing in pytest.ini as it was creating the wrong report for coveralls

* conftest.py, fixtures: added docstrings

* concretize_preferences.py: uses fixtures instead of subclassing MockPackagesTest

* directory_layout.py: uses fixtures instead of subclassing MockPackagesTest

* install.py: uses fixtures instead of subclassing MockPackagesTest

* optional_deps.py: uses fixtures instead of subclassing MockPackagesTest

optional_deps.py: uses fixtures instead of subclassing MockPackagesTest

* packages.py: uses fixtures instead of subclassing MockPackagesTest

* provider_index.py: uses fixtures instead of subclassing MockPackagesTest

* spec_yaml.py: uses fixtures instead of subclassing MockPackagesTest

* multimethod.py: uses fixtures instead of subclassing MockPackagesTest

* install.py: now uses mock_archive_url

* git_fetch.py: uses fixtures instead of subclassing MockPackagesTest

* hg_fetch.py: uses fixtures instead of subclassing MockPackagesTest

* svn_fetch.py, mirror.py: uses fixtures instead of subclassing MockPackagesTest
repo.py: deleted

* test_compiler_cmd.py: uses fixtures instead of subclassing MockPackagesTest

* cmd/module.py, cmd/uninstall.py: uses fixtures instead of subclassing MockDatabase

* database.py: uses fixtures instead of subclassing MockDatabase, removed mock/database

* pytest: uncluttering fixture implementations

* database: changing the scope to 'module'

* config.py: uses fixtures instead of subclassing MockPackagesTest

* spec_dag.py, spec_semantics.py: uses fixtures instead of subclassing MockPackagesTest

* stage.py: uses fixtures instead of subclassing MockPackagesTest. Removed mock directory

* pytest: added docstrings to all the fixtures

* pytest: final cleanup

* build_system_guess.py: fixed naming and docstrings as suggested by @scheibelp

* spec_syntax.py: added expected failure on parsing multiple specs closes #1976

* Add pytest and pytest-cov to Spack externals.

* Make `spack flake8` ignore externals.

* run-unit-tests runs spack test and not pytest.

* Remove all the special stuff for `spack test`

- Remove `conftest.py` magic and all the special case stuff in `bin/spack`

- Spack commands can optionally take unknown arguments, if they want to
  handle them.

- `spack test` is now a command like the others.

- `spack test` now just delegates its arguments to `pytest`, but it does
  it by receiving unknown arguments and NOT taking an explicit
  help argument.

* Fix error in fixtures.

* Improve `spack test` command a bit.

- Now supports an approximation of the old simple interface
- Also supports full pytest options if you want them.

* Use external coverage instead of pytest-cov

* Make coverage use parallel-mode.

* change __init__.py docs to include pytest

share/spack/qa/changed_files

@@ -1,31 +0,0 @@
-#!/usr/bin/env bash
-#
-# Description:
-#     Returns a list of changed files.
-#
-# Usage:
-#     changed_files [<directory> ...]
-#     changed_files [<file> ...]
-#     changed_files ["*.<extension>" ...]
-#
-# Options:
-#     Directories, files, or globs to search for changed files.
-#
-
-# Move to root directory of Spack
-# Allows script to be run from anywhere
-SPACK_ROOT="$(dirname "$0")/../../.."
-cd "$SPACK_ROOT"
-
-# Add changed files that have been committed since branching off of develop
-changed=($(git diff --name-only --diff-filter=ACMR develop... -- "$@"))
-# Add changed files that have been staged but not yet committed
-changed+=($(git diff --name-only --diff-filter=ACMR --cached -- "$@"))
-# Add changed files that are unstaged
-changed+=($(git diff --name-only --diff-filter=ACMR -- "$@"))
-# Add new files that are untracked
-changed+=($(git ls-files --exclude-standard --other -- "$@"))
-
-# Return array
-# Ensure that each file in the array is unique
-printf '%s\n' "${changed[@]}" | sort -u

share/spack/qa/run-unit-tests

@@ -43,4 +43,9 @@ spack config get compilers
 spack install -v libdwarf
 
 # Run unit tests with code coverage
-coverage run bin/spack test "$@"
+if [[ "$TRAVIS_PYTHON_VERSION" == 2.7 ]]; then
+    coverage run bin/spack test "$@"
+    coverage combine
+else
+    spack test "$@"
+fi