Filter out internal object prefix during listing (#4435)

We use ZZZZ-Minio/ prefix internally in our GCS gateway which should be
filtered out in the response to ListObjects.

cmd/gateway-gcs_test.go

@@ -129,3 +129,38 @@ func TestValidGCSProjectID(t *testing.T) {
 		}
 	}
 }
+
+// Test for isGCSPrefix
+func TestIsGCSPrefix(t *testing.T) {
+	testCases := []struct {
+		prefix      string
+		expectedRes bool
+	}{
+		// Regular prefix without a trailing slash
+		{
+			prefix:      "hello",
+			expectedRes: false,
+		},
+		// Regular prefix with a trailing slash
+		{
+			prefix:      "hello/",
+			expectedRes: false,
+		},
+		// GCS prefix without a trailing slash
+		{
+			prefix:      ZZZZMinioPrefix,
+			expectedRes: true,
+		},
+		// GCS prefix with a trailing slash
+		{
+			prefix:      ZZZZMinioPrefix + "/",
+			expectedRes: true,
+		},
+	}
+
+	for i, tc := range testCases {
+		if actualRes := isGCSPrefix(tc.prefix); actualRes != tc.expectedRes {
+			t.Errorf("%d: Expected isGCSPrefix to return %v but got %v", i, tc.expectedRes, actualRes)
+		}
+	}
+}