Do not output XFAIL/XPASS.

The parser for treehugger does not understand these output values and
they are hard to understand.

Also, add a list description to make it clear what tests are failing and
why.

Test: Unit tests pass.
Test: Verify that bionic unit tests pass in treehugger.
Change-Id: I7911f96c5e7f9270ea97038435502bbd79d65d66