Fix global variable collision in gdb.multi/multi-kill.exp

Fix global variable collision in gdb.multi/multi-kill.exp

The new gdb.multi/multi-kill.exp testcase added an 'testpid' array,
which may conflict with other global 'testpid' variables used by other
testcases, resulting in:

 ...
 ERROR: tcl error sourcing
 /data/gdb_versions/devel/src/gdb/testsuite/gdb.multi/multi-kill.exp.
 ERROR: can't set "testpid(1)": variable isn't array
     while executing
 "set testpid($num) [get_integer_valueof "pid" -1]"

or

 $ runtest gdb.threads/check-libthread-db.exp gdb.multi/multi-kill.exp
 ...
 Running /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.multi/multi-kill.exp ...
 Running /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.threads/check-libthread-db.exp ...
 ERROR: tcl error sourcing /home/pedro/gdb/binutils-gdb/src/gdb/testsuite/gdb.threads/check-libthread-db.exp.
 ERROR: can't set "testpid": variable is array
     while executing
 "set testpid [spawn_id_get_pid $test_spawn_id]"
     ("uplevel" body line 8)

Fix this with a namespace, like gdb.linespec/explicit.exp does.

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

	* gdb.multi/multi-kill.exp: Wrap in namespace.
	(start_inferior): Add TESTPID parameter.  Use it instead of the
	testpid global.
	(top level): Define empty TESTPID array, and pass it down to
	start_inferior.