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/genrule/genrule.go b/genrule/genrule.go
index e3823c5..f19e2aa 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -102,8 +102,9 @@
 
 	taskGenerator taskFunc
 
-	deps android.Paths
-	rule blueprint.Rule
+	deps       android.Paths
+	rule       blueprint.Rule
+	rawCommand string
 
 	exportedIncludeDirs android.Paths
 
@@ -287,6 +288,7 @@
 	genDir := android.PathForModuleGen(ctx)
 	// Escape the command for the shell
 	rawCommand = "'" + strings.Replace(rawCommand, "'", `'\''`, -1) + "'"
+	g.rawCommand = rawCommand
 	sandboxCommand := fmt.Sprintf("$sboxCmd --sandbox-path %s --output-root %s -c %s %s $allouts",
 		sandboxPath, genDir, rawCommand, depfilePlaceholder)