Add "signal SIGTRAP" test

Add "signal SIGTRAP" test

Some local changes I was working on related to SIGTRAP handling
resulted in "signal SIGTRAP" no longer passing the SIGTRAP to the
inferior.

Surprisingly, only annota1.exp catches this.  This commit adds a test
that doesn't rely on annotations, so that at the point annotations are
finaly dropped, we still have this use case covered ...

This is a multi-threaded test to also exercise the case of first
needing to do a step-over before delivering the signal.

Tested on x86_64 Fedora 20, native, remote/extended-remote gdbserver.

gdb/testsuite/
2015-02-10  Pedro Alves  <palves@redhat.com>

	* gdb.threads/signal-sigtrap.c: New file.
	* gdb.threads/signal-sigtrap.exp: New file.