refactor: move pytest.ini and top-level conftest.py to lib/spack/spack/test

- removes two files from root of repository
- `spack test` still works fine to run tests

pytest.ini

@@ -1,9 +0,0 @@
-# content of pytest.ini
-[pytest]
-addopts = --durations=20 -ra
-testpaths = lib/spack/spack/test
-python_files = *.py
-markers =
-  db: tests that require creating a DB
-  network: tests that require access to the network
-  maybeslow: tests that may be slow (e.g. access a lot the filesystem, etc.)