test: convert assertion that always fails to assert.fail()

In test-http-destroyed-socket-write2, the assert.strictEqual() in the
default case of the switch statement will always fail because it checks
for a value that is already accounted for in one of the switch cases.
Convert it to assert.fail().

PR-URL: #34793
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-destroyed-socket-write2.js

@@ -64,9 +64,7 @@ server.listen(0, function() {
 
       default:
         // Write to a torn down client should RESET or ABORT
-        assert.strictEqual(er.code,
-                           'ECONNRESET');
-        break;
+        assert.fail(`Unexpected error code ${er.code}`);
     }