Handle SIGSEGV in gdb selftests

The gdb.gdb self-tests were timing out for me, which turned out to be
PR testsuite/29325.  Looking into it, the problem is that the version
of the Boehm GC that is used by Guile on my machine causes a SEGV
during stack probing.  This unexpected stop confuses the tests and
causes repeated timeouts.

This patch adapts the two failing tests.  This makes them work for me,
and reduces the running time of gdb.gdb from 20 minutes to about 11
seconds.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29325