[gdb/testsuite] Don't run to main in gdb.cp/cplusfuncs.exp

[gdb/testsuite] Don't run to main in gdb.cp/cplusfuncs.exp

After building gdb with -fsanitize=threads, and running test-case
gdb.cp/cplusfuncs.exp, I run into a single timeout:
...
FAIL: gdb.cp/cplusfuncs.exp: info function operator=( (timeout)
...
and the test-case takes 2m33s to finish.

This is due to expanding CUs from libstdc++.

After de-installing package libstdc++6-debuginfo, the timeout disappears and
testing time goes down to 9 seconds.

Fix this by not running to main, which brings testing time down to 3 seconds.

With a gdb built without -fsanitize=threads, testing time goes down from 11
seconds to less than 1 second.

Tested on x86_64-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>