tests: more tweaks to help tests run in more envs

This is a pile of additional tweaks to the tests script to improve the ability to run the self-tests in more diverse environments.

1. in homebrew's test system, GIT_AUTHOR_NAME and GIT_AUTHOR_EMAIL are also set, which breaks the tests, so be sure to unset those as well
2. use readlink -f instead of -e, because while it's slightly less safe, it's supported by both the gnu readlink and bsd/macOS readlink
3. add support for specifying the patchpal binary to use for testing, so we can use an arbitrary one, rather than only the cargo build result

Signed-off-by: Jarod Wilson jarod@redhat.com