break out repo split and add unit test

cmd/drone-docker-ecr/main_test.go

@@ -0,0 +1,19 @@
+package main
+
+import "testing"
+
+func TestGetRepoName(t *testing.T) {
+	// map full repo path to expected repo name
+	repos := map[string]string{
+		"000000000000.dkr.ecr.us-east-1.amazonaws.com/repo":                     "repo",
+		"000000000000.dkr.ecr.us-east-1.amazonaws.com/namespace/repo":           "namespace/repo",
+		"000000000000.dkr.ecr.us-east-1.amazonaws.com/namespace/namespace/repo": "namespace/namespace/repo",
+	}
+
+	for repo, name := range repos {
+		splitName := getRepoName(repo)
+		if splitName != name {
+			t.Errorf("%s is not equal to %s.", splitName, name)
+		}
+	}
+}