kunit: Do not pollute source directory with generated files (test.log)

When --build_dir is provided use it and do not pollute source directory
which even can be mounted over network or read-only.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>