Fixed top frame assumption in watchpoint-hw-attach

watchpoint-hw-attach.exp was noticed to fail on some machines.
Thanks to the input from sergiodj and palves on the IRC channel,
it was concluded that the test case incorrectly assumed that on
attach it was landed in the top-most frame of the inferior. This
was fixed by running to a break point in main by explicitly
defining the source file name before continuing with the test.

Tested on the following architectures x86_64, aarch64 and ppc64le.

gdb/testsuite/ChangeLog:

	* gdb.base/watchpoint-hw-attach.c (main): Remove unneeded
	code.
	* gdb.base/watchpoint-hw-attach.exp: Break in outermost frame.