Port Test for increment with merge (#1547)

Ports tests from https://github.com/firebase/firebase-js-sdk/pull/3048/files

Author: schmidt-sebastian

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/NumericTransformsTest.java

@@ -95,6 +95,14 @@ public void mergeOnNonExistingDocumentWithIncrement() {
     expectLocalAndRemoteValue(1337L);
   }
 
+  @Test
+  public void mergeOnExistingDocumentWithIncrement() {
+    waitFor(docRef.set(map("sum", 1)));
+    expectLocalAndRemoteValue(1);
+    waitFor(docRef.set(map("sum", FieldValue.increment(1337)), SetOptions.merge()));
+    expectLocalAndRemoteValue(1338L);
+  }
+
   @Test
   public void integerIncrementWithExistingInteger() {
     writeInitialData(map("sum", 1337L));