ScoreTests.cs (created + tests added)

Tests added:
TestToString()
TestEquals_True()
TestEquals_False_ArgumentIsNull()
TestEquals_False_DifferentAttempts()
TestEquals_False_DifferentNicknames()
TestGetHashCode_EqualCodes()
TestGetHashCode_DifferentCodes_DifferentNicknames()
TestGetHashCode_DifferentCodes_DifferentAttempts()