Unit testing a method that calls another method

modify(string value)
{
    if(value.Length > 5)  replaceit(value);

    else changeit(value);
}

public TestModifyIfValueLength..()
    {
      string expectedValue = .. ;// literal result of replaceit(value)
      Assert.Equals( expectedValue, modify("asd") );
    }

testLongValue() {
    string testValue = "A value longer than 5 chars";
    string expected = "Replaced!";
    string actual = modify(testValue);
    assertEqual(expected, actual);
}

testShortValue() {
    string testValue = "len4";
    string expected = "Changed!";
    string actual = modify(testValue);
    assertEqual(expected, actual);
}