[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
diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index c08491f..0927032 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -170,7 +170,7 @@
     group.add_argument('--wait', action='store_true',
                        help='wait for user input, e.g., allow a debugger '
                             'to be attached')
-    group.add_argument('--slaveargs', metavar='ARGS')
+    group.add_argument('--worker-args', metavar='ARGS')
     group.add_argument('-S', '--start', metavar='START',
                        help='the name of the test at which to start.' +
                             more_details)