[committed, obvious] gdb/testsuite: Fix TUI support checks in gdb.tui tests

[Sandra Loosemore]

I was seeing a bunch of FAILs and ERRORs in gdb.tui tests in a 
remote-host test configuration that doesn't support TUI.   It turns out 
all these tests are checking for TUI support, but only printing an 
UNSUPPORTED message and not doing anything to skip the test.  I'm sure 
that's not intentional since the tests can't possibly work in this 
situation.  I've committed this patch to make all these tests return 
early when the TUI support check fails.

-Sandra
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