selftest: allow '-R' and '-r' be used together

For now, there's no easy way to skip some specified tests when
using 'oe-selftest -r'. This is because '-R' and '-r' are in the
mutually exclusive group. Looking at the codes, the codes have already
allowed running specified tests and skipping specified tests at
the same time. So move '-R' out of the mutually exclusive group
and change its help message.

After this change, the following command could be used.
  oe-selftest -r A -R A.B.C

This does introduce a behavior regression.
Previous 'oe-selftest -R xxx' needs to now be changed to
'oe-selftest -a -R xxx'.

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