Cygwin/testsuite: Avoid infinite hang

On Cygwin, the gdb.base/fork-no-detach-follow-child-dlopen.exp
testcase hits a sequence of cascading FAILs:

 (gdb) run
 Starting program: ..../gdb.base/fork-no-detach-follow-child-dlopen/fork-no-detach-follow-child-dlopen
 [New Thread 12672.0x318c]
 [New Thread 12672.0x2844]
 [New Thread 12672.0x714]
 FAIL: gdb.base/fork-no-detach-follow-child-dlopen.exp: runto: run to add (timeout)
 frame
 FAIL: gdb.base/fork-no-detach-follow-child-dlopen.exp: frame (timeout)
 list
 FAIL: gdb.base/fork-no-detach-follow-child-dlopen.exp: list (timeout)

And the test program never makes progress.

... and at this point, Cygwin is completely stuck.  I can't run any
other Cygwin program.

However, if we run the test program outside DejaGnu, we see something
different:

  (gdb) b add
  Function "add" not defined.
  Make breakpoint pending on future shared library load? (y or [n]) y
  Breakpoint 1 (add) pending.
  (gdb) r
  Starting program: ..../gdb.base/fork-no-detach-follow-child-dlopen/fork-no-detach-follow-child-dlopen
  [New Thread 10968.0x834]
  [New Thread 10968.0x29a4]
  [New Thread 10968.0x16b8]
  [New Thread 10968.0xf9c]
  [Switching to Thread 10968.0x16b8]

  Thread 4 "sig" hit Breakpoint 1.2, pending_signals::add (pack=..., this=0x7ffa1e748a40 <sigq>) at /usr/src/debug/cygwin-3.4.9-1/winsup/cygwin/sigproc.cc:1304
  1304      se = sigs + pack.si.si_signo;
  (gdb)

Ah, the test wanted to run to a global "add" function, but managed to
stop at an internal Cygwin method called "add".  And stopping there
deadlocks everything Cygwin in the system.  (I believe some
cygwin1.dll mechanisms use cross-process synchronization or
communication, we're probably blocking something like that.)

Fix this by using "break -q".  The tests FAIL because we don't support
follow-fork for Cygwin, but at least we no longer deadlock the
machine.

Approved-by: Kevin Buettner <kevinb@redhat.com>
Change-Id: I7181d8481c2ae1024b0d73e3bb194f9a4f0a7eb9