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 <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: Trott

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,