Closes#2990 ty @Atrius97 !
Remove the `object Content` accessor only used for Mystery Gifts (used to trickle up the PKM object since the two ribbon interfaces weren't implemented on the IEncounterable). Just make PCD/PGT implement the ribbon interfaces and delegate the get/set to PKM directly.
Rewrite the national ribbon check for clarity
Optimize invalid/missing ribbon string replace to operate on the final string rather than do linq and replace each input. With this we make 1 temp string only, rather than 1-per-ribbon.
Replace hardcoded "Ribbon" strings to use a shared const string in a central spot.
* Rewrite tests with XUnit and .Net Core
* Add better "because" message
* Skipping test that was not ready & convert the fact to a theory
* Tweak casing
* Convert select date tests to theories
* Make the GetStringList load lock safer
