Switch CheckSnapshot from a testSdkResult method to a function

This will allow the testSdkResult to be replaced with the TestResult
when switching sdk package to use test fixtures.

Bug: 181070625
Test: m nothing
Change-Id: Ieca63f4c189f5e804102aeb4073289ea03143b6e
diff --git a/sdk/sdk_test.go b/sdk/sdk_test.go
index a7acd0c..65a9001 100644
--- a/sdk/sdk_test.go
+++ b/sdk/sdk_test.go
@@ -169,7 +169,7 @@
 			"package/Android.bp": []byte(packageBp),
 		})
 
-	result.CheckSnapshot("mysdk", "package",
+	CheckSnapshot(result, "mysdk", "package",
 		checkAndroidBpContents(`
 // This is auto-generated. DO NOT EDIT.
 
@@ -317,7 +317,7 @@
 	`
 	result := testSdkWithFs(t, sdk, nil)
 
-	result.CheckSnapshot("mysdk", "",
+	CheckSnapshot(result, "mysdk", "",
 		checkAllOtherCopyRules(`.intermediates/mysdk/common_os/mysdk-current.zip -> mysdk-current.zip`),
 	)
 }