Increase coverage of spack.relocate (#16475)

- add docstrings and make parameter names consistent in `relocate.py`
- Make `replace_prefix_*` and other functions private (as they are implementation details)
- remove unused function _replace_prefix_nullterm()

- Add unit tests for `relocate.py` functions
  - add patchelf to Travis and use it during tests
  - add hello_world fixture with a compiled binary, so we can test relocation

.travis.yml

@@ -32,6 +32,25 @@ jobs:
       dist: trusty
       os: linux
       language: python
+      addons:
+        apt:
+          # Everything but patchelf, that is not available for trusty
+          packages:
+            - ccache
+            - cmake
+            - gfortran
+            - graphviz
+            - gnupg2
+            - kcov
+            - mercurial
+            - ninja-build
+            - perl
+            - perl-base
+            - realpath
+            - r-base
+            - r-base-core
+            - r-base-dev
+            - zsh
       env: [ TEST_SUITE=unit, COVERAGE=true ]
     - python: '2.7'
       os: linux
@@ -84,6 +103,7 @@ addons:
       - kcov
       - mercurial
       - ninja-build
+      - patchelf
       - perl
       - perl-base
       - realpath