Add tests for genrule command expansion

Add tests for expanding variables in a genrule cmd property.

Test: genrule_test.go
Change-Id: I8288b8616d518bb5f24a892c4e59f68d95055d0a

android/config.go

@@ -241,6 +241,8 @@ func TestArchConfig(buildDir string, env map[string]string) Config {
 		},
 	}
 
+	config.BuildOsVariant = config.Targets[Host][0].String()
+
 	return testConfig
 }