test: improve generic type assertion test

The test added in CL 420394 only tested that the type assertions
compiled at all. This CL changes it into a run test to make sure the
type assertions compile and also run correctly.

Updates #54135.

Change-Id: Id17469faad1bb55ff79b0bb4163ef50179330033
Reviewed-on: https://go-review.googlesource.com/c/go/+/420421
Run-TryBot: Matthew Dempsky <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: mdempsky

test/typeparam/issue54135.go

@@ -1,4 +1,4 @@
-// compile
+// run
 
 // Copyright 2022 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -19,8 +19,12 @@ type Baz interface {
 }
 
 func check[T comparable](p Bar[T]) {
-	_, _ = p.(any)
-	_, _ = p.(Baz)
+	if x, ok := p.(any); !ok || x != p {
+		panic("FAIL")
+	}
+	if _, ok := p.(Baz); ok {
+		panic("FAIL")
+	}
 }
 
 func main() {