patman: Provide option to ignore bad aliases

Often it happens that patches include tags which don't have aliases. It
is annoying that patman fails in this case, and provides no option to
continue other than adding empty tags to the .patman file.

Correct this by adding a '-t' option to ignore tags that don't exist.
Print a warning instead.

Since running the tests is not a common operation, move this to --test
instead, to reserve -t for this new option.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>