test: move test-tls-securepair-client out of pummel

test-tls-securepair-client does not seem to need to be in the pummel
directory. Move it to sequential. (It can't go into parallel because it
uses common.PORT and therefore might conflict with another test that is
using system-assigned available ports.)

PR-URL: #25222
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Denys Otrishko <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott