gopls/internal/regtest: eliminate DiagnosticAtRegexp

Replace DiagnosticAtRegexp with diagnostic filters. This allowed
eliminating DiagnosticExpectation, which was the only complicated
implementation of the Expectation interface. Replace Expectation with
the concrete (formerly named) SimpleExpectation.

Updates golang/go#39384

Change-Id: I6716e869609dce9777025557494c8f81a606e4ff
Reviewed-on: https://go-review.googlesource.com/c/tools/+/461939
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>

gopls/internal/lsp/regtest/env.go

@@ -297,7 +297,7 @@ func checkExpectations(s State, expectations []Expectation) (Verdict, string) {
 		if v > finalVerdict {
 			finalVerdict = v
 		}
-		summary.WriteString(fmt.Sprintf("%v: %s\n", v, e.Description()))
+		summary.WriteString(fmt.Sprintf("%v: %s\n", v, e.Description))
 	}
 	return finalVerdict, summary.String()
 }