gdb: test vfork + follow-fork-mode=parent + detach-on-fork=off

The particular behavior we have when using that combination of settings
doesn't seem tested at all (at least, I don't find it if I grep for "Can
not resume the parent process").  Add a simple test for that.

Change-Id: Ib9454a615abba661b42f1b15056df73ed1bcd4c5