releasetools: Add sanity test for sign_apex.

This ensures a matching interface between sign_apex and apex_utils.

The test apex `testdata/foo.apex` is generated by running
`system/apex/apexer/runtests.sh`.

Test: python -m unittest test_sign_apex
Test: atest releasetools_test
Change-Id: I7c14b1df2a3038ad206aa3e5aac084c47baaa00b