Reorganize fossil tests (#99)

Author: FZambia

lib/src/fossil_test.dart renamed to test/fossil_test.dart

@@ -1,7 +1,9 @@
 import 'dart:io';
 import 'dart:typed_data';
+
+// WARNING: Internal test, do not rely on fossil.dart outside this scope.
+import 'package:centrifuge/src/fossil.dart';
 import 'package:test/test.dart';
-import 'fossil.dart';
 
 Uint8List readBytes(String path) {
   final file = File(path);
@@ -16,7 +18,7 @@ void main() {
     // There are test cases in testdata/fossil/1 through 5
     for (var i = 1; i <= 5; i++) {
       test('Delta case #$i', () {
-        final baseDir = 'testdata/fossil/$i';
+        final baseDir = 'test/testdata/fossil/$i';
         final originPath = '$baseDir/origin';
         final deltaPath = '$baseDir/delta';
         final targetPath = '$baseDir/target';