gdb/testsuite: gdb.base/continue-all-already-running.exp: add fail if can't run to...

gdb/testsuite: gdb.base/continue-all-already-running.exp: add fail if can't run to main

While doing some changes, I managed to break this test in a way that
running to main didn't work.  However, it didn't produce any FAIL.  I
noticed because I diff'ed the results and saw some PASSes unexpectedly
disappear, but that's a bit fragile.  Add a fail in case this test fails
to run to main.  Ideally, I think that runto_main should by default
produce a FAIL when it fails (the opposite of the existing logic), but
that's a project of its own, so just fix this test for the moment.

gdb/testsuite/ChangeLog:

	* gdb.base/continue-all-already-running.exp: Call fail if can't
	run to main.

Change-Id: I84b816a126c92ac579ed5ebbe39b017bd5cb7096