[3.7] bpo-34279: Synchronize regrtest with master (GH-10800)

[3.7] bpo-34279: Synchronize regrtest with master (GH-10800)

* bpo-34605, libregrtest: Rename --slaveargs to --worker-args (GH-9099)

Rename also run_tests_slave() to run_tests_worker().

(cherry picked from commit 012f5b968a738b15ae9b40c499a1c0778b0615a9)

* bpo-34279, regrtest: Issue a warning if no tests have been executed (GH-10150)

(cherry picked from commit 9724348b43a9005a449ba532ccd3c6726f031097)

* test_regrtest: remove unused threading import