localrpc test: Don't call t.Fatal from a goroutine

The testing package does not allow t.Fatal to be called from a different
goroutine; however, we do that if the testing server fails listen or
accept connections.

Since that is unexpected and rare, this patch turns those calls into
panics.

internal/localrpc/client_test.go

@@ -15,13 +15,13 @@ func NewFakeServer(t *testing.T, path, output string) {
 	t.Helper()
 	lis, err := net.Listen("unix", path)
 	if err != nil {
-		t.Fatal(err)
+		panic(err)
 	}
 
 	for {
 		conn, err := lis.Accept()
 		if err != nil {
-			t.Fatal(err)
+			panic(err)
 		}
 		t.Logf("FakeServer %v: accepted ", conn)