Expand PriorityQueue test coverage (#48420)

* make comparer field readonly

* abstract IComparer in PriorityQueue generic tests

does minor refactorings and adds new tests

* add PriorityQueue property tests

* address feedback

* add seed for deterministic runs

* apply var fixes in property tests

* replace FsCheck properties with theories

Author: eiriktsarpalis

src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs

@@ -23,7 +23,7 @@ public class PriorityQueue<TElement, TPriority>
         /// <summary>
         /// Custom comparer used to order the heap.
         /// </summary>
-        private IComparer<TPriority>? _comparer;
+        private readonly IComparer<TPriority>? _comparer;
 
         /// <summary>
         /// Lazily-initialized collection used to expose the contents of the queue.