RankListTests.cs added + file paths remade

RankListTests.cs (created + tests added)
public void TestLoadFromFile_ShouldLoad_OneScore()
public void TestLoadFromFile_ShouldLoad_TwoScores()
public void TestLoadFromFile_ShouldLoad_FiveScores()
These methods are tested with test_load_1.txt, test_load_2.txt,
test_load_5.txt files

SecretNumberTests.cs (flag renamed to isInRange)

Engine.cs
this.topScores.LoadFromFile("scores.txt") file path fixed to
this.topScores.LoadFromFile(@"../../scores.txt")