Disable TestVariantSingletonModule when go test -short is used

TestVariantSingletonModule tests that an error is produced when a
mutator creates variants of a SingletonModule.  It also triggers
a data race when cloneLogicModules copies the properties, as the
ModuleFactory of a SingletonModule always returns the same module
with the same property structs.  The build is about to fail with an
error anyways, so hopefully the data race isn't a problem.  There
is no way to disable the test when the data race detector is enabled,
so disable it when -short is enabled, which is the case for all of
our CI testing.

Test: go test -race -short ./...
Change-Id: I944a4901acb60df8e590270f178961d95095a7e9
diff --git a/android/singleton_module_test.go b/android/singleton_module_test.go
index 9d98478..3b1bf39 100644
--- a/android/singleton_module_test.go
+++ b/android/singleton_module_test.go
@@ -103,6 +103,9 @@
 }
 
 func TestVariantSingletonModule(t *testing.T) {
+	if testing.Short() {
+		t.Skip("test fails with data race enabled")
+	}
 	bp := `
 		test_singleton_module {
 			name: "test_singleton_module",