test: use fixtures.readKey

Replace the common.fixturesDir by fixtures.readKey
in http2 subsystem test 'server startup'.

PR-URL: https://github.com/nodejs/node/pull/15892


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>