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
diff --git a/tools/releasetools/Android.bp b/tools/releasetools/Android.bp
index b5ae009..8cf3fab 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -44,6 +44,7 @@
         "ota_from_target_files.py",
         "ota_package_parser.py",
         "rangelib.py",
+        "sign_apex.py",
         "sign_target_files_apks.py",
         "sparse_img.py",
         "target_files_diff.py",