kunit: tool: fix running kunit_tool from outside kernel tree

Currently kunit_tool does not work correctly when executed from a path
outside of the kernel tree, so make sure that the current working
directory is correct and the kunit_dir is properly initialized before
running.

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