Add tests for genrule command expansion

Add tests for expanding variables in a genrule cmd property.

Test: genrule_test.go
Change-Id: I8288b8616d518bb5f24a892c4e59f68d95055d0a
diff --git a/Android.bp b/Android.bp
index ac50166..ccab727 100644
--- a/Android.bp
+++ b/Android.bp
@@ -192,6 +192,9 @@
     srcs: [
         "genrule/genrule.go",
     ],
+    testSrcs: [
+        "genrule/genrule_test.go",
+    ],
     pluginFor: ["soong_build"],
 }