test: reset a signal handler before closing a signal channel

goroutine 5 [running]:
os/signal.process(0x10e21c0, 0xc00050c280)
        third_party/go/gc/src/os/signal/signal.go:227 +0x164
os/signal.loop()
        third_party/go/gc/src/os/signal/signal_unix.go:23 +0x3e
created by os/signal.init.0
        third_party/go/gc/src/os/signal/signal_unix.go:29 +0x41

PiperOrigin-RevId: 264518530

test/syscalls/syscall_test_runner.go

@@ -280,6 +280,7 @@ func runTestCaseRunsc(testBin string, tc gtest.TestCase, t *testing.T) {
 	if err = cmd.Run(); err != nil {
 		t.Errorf("test %q exited with status %v, want 0", tc.FullName(), err)
 	}
+	signal.Stop(sig)
 	close(sig)
 }