Add StringExtensions Unit Tests

[hishamco]

In favor of #14068

[hishamco]

Seems there are some extensions need to be removed (unused) or replaced by native APIs

[hishamco]

Strange!! the build failed for unrelated issue

@MikeAlhayek any idea?

[MikeAlhayek]

This is a wrong logic

if (chars == null || chars.Length == 0)
{
return false;
}

[hishamco]

Why?!! the old logic was wrong, when source is null it should returns false not true

[MikeAlhayek]

we'll that's different that the previous logic. It could be one of the reason the tests are failing.

[hishamco]

There was no test before, and that is why the test failed when I use the previous logic

[Piedone]

Do you want to get back to reviewing this, @MikeAlhayek?

[hishamco]

@MikeAlhayek Anything to add here or shall I merge this?