[testsuite, obvious] Add check for readline support to more GDB tab-completion tests.

[Sandra Loosemore]

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

4ee9b0c53a8055b869dfe424846bc00eb0cd0c8f
515b89b30c18dae6b91e57ec2dd336919bb4578d

-Sandra
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