chore(windows): tests should be case-sensitive

DUnitX by default does case-insensitive string equality assertions, and
we don't really want to make that assumption.

There was one minor fail uncovered in Keyman.System.UILanguageManager as
a result of this -- it now returns a matching entry as found in the
list of UI languages, rather than the input, to get same case as the UI
language list entry.

Thank you @ermshiperete.