test: fix test-tls-securepair-client
test-tls-securepair-client has been failing for over a year but no one noticed because pummel tests are almost never run.
In preparation for running pummel tests once a day in CI, fix test-tls-securepair-client.
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.)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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