bpo-36725: Refactor regrtest multiprocessing code (GH-12961)

Rewrite run_tests_multiprocess() function as a new MultiprocessRunner
class with multiple methods to better report errors and stop
immediately when needed.

Changes:

* Worker processes are now killed immediately if tests are
  interrupted or if a test does crash (CHILD_ERROR): worker
  processes are killed.
* Rewrite how errors in a worker thread are reported to
  the main thread. No longer ignore BaseException or parsing errors
  silently.
* Remove 'finished' variable: use worker.is_alive() instead
* Always compute omitted tests. Add Regrtest.get_executed() method.

Lib/test/libregrtest/runtest.py

@@ -275,6 +275,7 @@ def _runtest_inner(ns, test_name, display_failure=True):
     except support.TestDidNotRun:
         return TEST_DID_NOT_RUN
     except KeyboardInterrupt:
+        print()
         return INTERRUPTED
     except:
         if not ns.pgo: