go/types, types2: simplify TestUnaliasTooSoonInCycle (cleanup)

Follow-up on CL 576975 and CL 579015.

Updates #66704
Updates #65294

Change-Id: Ied95386a346be38ccda86d332d09b2089a68c5e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/579075
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/api_test.go

@@ -2997,14 +2997,9 @@ type T[_ any] struct{}
 type A T[B]
 type B = T[A]
 `
-
-	f := mustParse(src)
-	pkg, err := new(Config).Check("a", []*syntax.File{f}, nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	pkg := mustTypecheck(src, nil, nil)
 	B := pkg.Scope().Lookup("B")
+
 	got, want := Unalias(B.Type()).String(), "a.T[a.A]"
 	if got != want {
 		t.Errorf("Unalias(type B = T[A]) = %q, want %q", got, want)

src/go/types/api_test.go

@@ -3005,17 +3005,9 @@ type T[_ any] struct{}
 type A T[B]
 type B = T[A]
 `
-	fset := token.NewFileSet()
-	f, err := parser.ParseFile(fset, "a.go", src, 0)
-	if err != nil {
-		t.Fatal(err)
-	}
-	pkg, err := new(Config).Check("a", fset, []*ast.File{f}, nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	pkg := mustTypecheck(src, nil, nil)
 	B := pkg.Scope().Lookup("B")
+
 	got, want := Unalias(B.Type()).String(), "a.T[a.A]"
 	if got != want {
 		t.Errorf("Unalias(type B = T[A]) = %q, want %q", got, want)