gdb/testsuite: accept script argument for mi_make_breakpoint_pending

This commit changes mi_make_breakpoint_pending to accept the 'script'
and 'times' arguments.

I've then added a new test that makes use of 'scripts' in
gdb.mi/mi-pending.exp and gdb.mi/mi-dprintf-pending.exp.

There is already a test in gdb.mi/mi-pending.exp that uses the 'times'
argument -- previously this argument was being ignored, but is now
used.

Reviewed-By: Tom Tromey <tom@tromey.com>