Readability improvements in ApfTest.

- Use "R0"/"R1" instead of "Register.R0"/"Register.R1".
- Include in the test failure message a command that can be used
  to debug the failure using apf_run.

Bug: 263429226
Test: test-only change
Change-Id: If280027193977943bc6a9e9c1bb5db41e8563b0a
1 file changed