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: #25188
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/pummel/test-tls-session-timeout.js

@@ -50,8 +50,8 @@ function doTest() {
 
  const SESSION_TIMEOUT = 1;
 
- const key = fixtures.path('agent.key');
- const cert = fixtures.path('agent.crt');
+ const key = fixtures.readSync('agent.key');
+ const cert = fixtures.readSync('agent.crt');
  const options = {
  key: key,
  cert: cert,