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: https://github.com/nodejs/node/pull/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>