Minor fix in skip_ctf_tests

I noticed an oddity in skip_ctf_tests -- for me it ends up caching the
string "!0", because it ends with 'return ![...]'.  In Tcl, this is
just string concatenation.

The result works because the users of this function have unbraced if
conditions, like:

    if [skip_ctf_tests] {

... which works because "if" re-parses the returned string as an
expression, and evaluates that.

There's only a latent bug here, but this is also un-idiomatic, so I am
checking in this patch to fix it.  This way, if someone in the future
uses a braced condition (which is what I normally recommend), it will
continue to work.

gdb/testsuite/ChangeLog
2021-02-26  Tom Tromey  <tom@tromey.com>

	* lib/gdb.exp (skip_ctf_tests): Use expr on result.