tests: cleanup: Explain the purpose of some tests

tests: cleanup: Explain the purpose of some tests

Tests explained:
    * test_everything() (see note below)
    * `include` test cases

Identifying the intended purpose of the `include` test cases is a
partial prerequisite for the future work of checking that each
configuration test expecting a failure fails for the intended reason. At
the momment, we only check that configuration fails regardless of the
reason.

Note: test_everything() is a single stand alone test case that dates
back to from barectf 2 before the `tracing` tests existed. This is why
it checks more than just configuration generation. It is possibly
redundant with the more extensive tracing tests.

Change-Id: I59fe258b4f1eb7ef5c83f717f12973d38269adb4
Signed-off-by: Erica Bugden <ebugden@efficios.com>