This extension makes testing easier, regarding populating the fake database. Also, it adds a convenient way for checking whether the database was successfully modified by another function.

cloud_firestore_mocks_extension.dart

import 'dart:convert';

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:cloud_firestore_mocks/cloud_firestore_mocks.dart';
import 'package:collection/collection.dart';

extension MockFirestoreTesting on MockFirestoreInstance {
  /// Populates the [MockFirestoreInstance] with data from a JSON-String.
  Future<void> populate(String jsonSource) async {
    final Map<String, dynamic> sourceMap = json.decode(jsonSource);
    for (var collectionEntry in sourceMap.entries) {
      await _populate('${collectionEntry.key}', collectionEntry.value);
    }
  }

  Future<void> _populate(String path, Map<String, dynamic> root) async {
    Map<String, dynamic> data = {};
    for (var entry in root.entries) {
      final value = entry.value;
      // If the value of the field is another map, run this function.
      if (value is Map<String, dynamic>) {
        _populate('$path/${entry.key}', value);
        continue;
      }
      // Here, the analysis of each field is done.
      if (value is String &&
          value.contains(
              RegExp(r'^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}$'))) {
        data[entry.key] = value.toTimestamp();
        continue;
      }
      data[entry.key] = value;
    }
    // If the data map is empty, this means that this root map is probably a collection.
    if (data.isNotEmpty) {
      await this.document('$path').setData(data, merge: true);
    }
  }

  /// Checks if the structure of the [MockFirestoreInstance] database matches with a given JSON-String.
  bool matchStructure(String jsonSource) {
    final Map<String, dynamic> rootMap = json.decode(this.dump());
    final Map<String, dynamic> jsonSourceMap = json.decode(jsonSource);
    print('Expected:\n$jsonSource\n');
    print('Actual:\n${this.dump()}\n');
    return DeepCollectionEquality.unordered().equals(rootMap, jsonSourceMap);
  }
}

extension Conversion on String {
  Timestamp toTimestamp() {
    int year = int.parse(this.substring(0, 4));
    int month = int.parse(this.substring(5, 7));
    int day = int.parse(this.substring(8, 10));
    int hour = int.parse(this.substring(11, 13));
    int minute = int.parse(this.substring(14, 16));
    int second = int.parse(this.substring(17, 19));
    int millisecond = int.parse(this.substring(20));
    return Timestamp.fromDate(
        DateTime(year, month, day, hour, minute, second, millisecond));
  }
}

If anyone has an idea for getting the difference between the expected and the actual and printing it, i would really appreciate it.