Fix foreach_with_prefix regression

Fix foreach_with_prefix regression

Fix a silly bug in commit a26c8de0ee93 ("Fix early return in
foreach_with_prefix").

That patch made foreach_with_prefix always return after the first
iteration, making ~10k tests disappear from test runs...

This fixes it, and as penance, adds a testcase that exercises all
kinds of different returns possible (ok, error, return, break,
continue).  I've written it with regular "foreach", and then switched
to foreach_with_prefix and made sure we get the same results.  I put
the testcase in a new gdb.testsuite/ subdir, since this is exercising
the testsuite harness bits.  We can move this elsewhere if people
prefer a different place, but I'm going ahead in order to unbreak the
testsuite ASAP.

gdb/testsuite/ChangeLog:
2019-07-04  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (foreach_with_prefix): Don't return early if
	body returned ok(0), break(3) or continue(4).
	* gdb.testsuite/foreach_with_prefix.exp: New file.