kunit: run kunit_tool from any directory

Implemented small fix so that the script changes work directories to the
root of the linux kernel source tree from which kunit.py is run. This
enables the user to run kunit from any working directory. Originally
considered using os.path.join but this is more error prone as we would
have to find all file path usages and modify them accordingly. Using
os.chdir ensures that the entire script is run within /linux.

Signed-off-by: Heidi Fahim <heidifahim@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>