kunit: tool: don't include KTAP headers and the like in the test log

We print the "test log" on failure.
This is meant to be all the kernel output that happened during the test.

But we also include the special KTAP lines in it, which are often
redundant.

E.g. we include the "not ok" line in the log, right before we print
that the test case failed...
[13:51:48] Expected 2 + 1 == 2, but
[13:51:48] 2 + 1 == 3 (0x3)
[13:51:48] not ok 1 example_simple_test
[13:51:48] [FAILED] example_simple_test

More full example after this patch:
[13:51:48] =================== example (4 subtests) ===================
[13:51:48] # example_simple_test: initializing
[13:51:48] # example_simple_test: EXPECTATION FAILED at lib/kunit/kunit-example-test.c:29
[13:51:48] Expected 2 + 1 == 2, but
[13:51:48] 2 + 1 == 3 (0x3)
[13:51:48] [FAILED] example_simple_test

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>