kunit: kunit_tool: Separate out config/build/exec/parse

Add new subcommands to kunit.py to allow stages of the existing 'run'
subcommand to be run independently:
- 'config': Verifies that .config is a subset of .kunitconfig
- 'build': Compiles a UML kernel for KUnit
- 'exec': Runs the kernel, and outputs the test results.
- 'parse': Parses test results from a file or stdin

The 'run' command continues to behave as before.

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