Improve Copy tests ensuring failures if there's not a deep copy

[hahn-kev]

Right now we have some tests for the copy method, however if the copy method does not make a deep copy of a Multistring the test will not fail. We can fix our tests to catch that case with the example seen here
fluentassertions/fluentassertions#2315 (comment)

For example this should fail

new Entry() {
LexemeForm = this.LexemeForm
}

because a copy of lexeme form was not made. This should pass

new Entry() {
LexemeForm = this.LexemeForm.Copy()
}

Since this is custom testing code there should be a test which asserts that the failure case throws