gdb.base/watch-bitfields.exp: Improve test

gdb.base/watch-bitfields.exp: Improve test

Make test messages unique and a couple other tweaks.

gdb/testsuite/
2014-09-16  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.base/watch-bitfields.exp: Pass string other than test file
	name to prepare_for_testing.
	(watch): New procedure.
	(expect_watchpoint): Use with_test_prefix.
	(top level): Factor out tests to ...
	(test_watch_location, test_regular_watch): ... these new
	procedures, and use with_test_prefix and gdb_continue_to_end.