Improve Assert.That error text message

[iamdmitrij]

Fixes #3767.

Assert.That for collections, error text changes to:

  Message: 
      Expected: exactly one item equal to "boop"
      But was:  **0 matching items** < "no-store, no-cache" >

by adding x matching item/s to the message text.

• Add matching keyword to error message
• Unit tests
• Unit test for no matching items missing use-case

[OsirisTerje]

lgtm