kunit: tool: make --raw_output support only showing kunit output

kunit: tool: make --raw_output support only showing kunit output

--raw_output is nice, but it would be nicer if could show only output
after KUnit tests have started.

So change the flag to allow specifying a string ('kunit').
Make it so `--raw_output` alone will default to `--raw_output=all` and
have the same original behavior.

Drop the small kunit_parser.raw_output() function since it feels wrong
to put it in "kunit_parser.py" when the point of it is to not parse
anything.

E.g.

$ ./tools/testing/kunit/kunit.py run --raw_output=kunit
...
[15:24:07] Starting KUnit Kernel ...
TAP version 14
1..1
    # Subtest: example
    1..3
    # example_simple_test: initializing
    ok 1 - example_simple_test
    # example_skip_test: initializing
    # example_skip_test: You should not see a line below.
    ok 2 - example_skip_test # SKIP this test should be skipped
    # example_mark_skipped_test: initializing
    # example_mark_skipped_test: You should see a line below.
    # example_mark_skipped_test: You should see this line.
    ok 3 - example_mark_skipped_test # SKIP this test should be skipped
ok 1 - example
[15:24:10] Elapsed time: 6.487s total, 0.001s configuring, 3.510s building, 0.000s running

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