context.py: avoid skipping tests by meaningless command argument

Currently `oe-selftest -R a' will skip 'archiver' tests. This is
not expected. Fix it so that the '-R' should be followed by actual
module/class/test names.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>