kunit: add coverage_uml.config to enable GCOV on UML

Now that kunit.py's --kunitconfig is repeatable, let's create a file to
hold the various options needed to enable coverage under UML.

This can be used like so:
$ ./tools/testing/kunit/kunit.py run \
  --kunitconfig=tools/testing/kunit/configs/all_tests_uml.config \
  --kunitconfig=tools/testing/kunit/configs/coverage_uml.config \
  --make_options=CC=/usr/bin/gcc-6

which on my system is enough to get coverage working [1].

This is still a clunky command, but far better than before.

[1] at the time of this commit, I get:
  Overall coverage rate:
    lines......: 11.6% (34112 of 295033 lines)
    functions..: 15.3% (3721 of 24368 functions)

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