test: move pummel tls test to sequential

Move test-tls-session-timeout from pummel to sequential. It isn't very
pummel-y and this will result in it being run on our CI more than once a
day. (It broke recently and it would have been caught if it was in
sequential rather than pummel.)

It must be in sequential rather than pummel because it uses
`common.PORT` which can result in test failures if more than one test
uses it at one time in parallel.

PR-URL: #26865
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: Trott