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/exports_test.go b/sdk/exports_test.go
index 1c59244..54a40d2 100644
--- a/sdk/exports_test.go
+++ b/sdk/exports_test.go
@@ -42,7 +42,7 @@
 			"package/Android.bp": []byte(packageBp),
 		})
 
-	result.CheckSnapshot("myexports", "package",
+	CheckSnapshot(result, "myexports", "package",
 		checkAndroidBpContents(`
 // This is auto-generated. DO NOT EDIT.