tests: each test client work on a distinct port range

apparently when running tests in parallel (the default of --clients 16),
there's a chance for two tests to use the same port.
specifically, one test might shutdown a master and still have the
replica up, and then another test will re-use the port number of master
for another master, and then that replica will connect to the master of
the other test.

this can cause a master to count too many full syncs and fail a test if
we run the tests with --single integration/psync2 --loop --stop

see Probmem 2 in #7314

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