[testsuite, obvious] Add check for readline support to more GDB tab-completion tests.
This patch makes the new-ish completion-support.exp infrastructure skip
tab-completion tests if there is no readline support. This fixes
several ERRORs we've run into while testing on remote Windows host via ssh.
I thought this was obvious enough to check in immediately since it is
consistent with some previous patches I've contributed to do similar
things, e.g. commits