Convert gdb.base/watchpoint-hw-attach.exp to spawn_wait_for_attach

Convert gdb.base/watchpoint-hw-attach.exp to spawn_wait_for_attach

On Cygwin, starting an inferior under GDB, and detaching it, quitting
GDB, and then closing the shell, like so:

  (gdb) start
  (gdb) detach
  (gdb) quit
  # close shell

... hangs the parent shell of GDB (not GDB!) until the inferior
process that was detached (as it is still using the same terminal GDB
was using) exits too.

This leads to odd failures in gdb.base/watchpoint-hw-attach.exp like
so:

 detach
 Detaching from program: .../outputs/gdb.base/watchpoint-hw-attach/watchpoint-hw-attach, process 16580
 [Inferior 1 (process 16580) detached]
 (gdb) FAIL: gdb.base/watchpoint-hw-attach.exp: detach

Fix this by converting the testcase to spawn the inferior outside GDB,
with spawn_wait_for_attach.

With this patch, the testcase passes cleanly on Cygwin, for me.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I8e3884073a510d6fd2fff611e1d26fc808adc4fa