V# generates incorrect test: an attempt to instantiate private class

* [V# version to reproduce](https://github.com/VSharp-team/VSharp/pull/228)
* [Test to reproduce](https://github.com/gsvgit/VSharp/blob/master/VSharp.Test/Tests/Algorithms.cs#L575)
* Steps
   * Run the test
   * Compile generated tests
   * Get a number of errors  "  GraphTests.cs(107, 34): [CS0122] 'Graph.Edge' is inaccessible due to its protection level"
