Fix gdb.multi/multi-kill.exp

Fix gdb.multi/multi-kill.exp

The previous patch misssed declaring the 'testpid' array as namespace
variable.  While it at, might as well go back to having start_inferior
refer to the "global" testpid, using "variable" too.

gdb/testsuite/ChangeLog:
2020-05-15  Pedro Alves  <palves@redhat.com>

	* gdb.multi/multi-kill.exp (start_inferior): Remove
	'testpid' parameter.  Refer to namespace variable directly.
	(testpid): Declare as namespace variable.