Add tests for genrule command expansion

Add tests for expanding variables in a genrule cmd property.

Test: genrule_test.go
Change-Id: I8288b8616d518bb5f24a892c4e59f68d95055d0a

Android.bp

@@ -192,6 +192,9 @@ bootstrap_go_package {
     srcs: [
         "genrule/genrule.go",
     ],
+    testSrcs: [
+        "genrule/genrule_test.go",
+    ],
     pluginFor: ["soong_build"],
 }