test: fix test-tls-session-timeout

Tests in pummel seem to break often and stay broken because they don't
get run in CI. In preparation for running pummel tests in CI once a day,
this fixes test-tls-session-timeout. `key` and `cert` are now the
contents of the relevant files and not the paths.

PR-URL: https://github.com/nodejs/node/pull/25188


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>