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/main.py

@@ -79,8 +79,8 @@ class Regrtest:
         self.skipped = []
         self.resource_denieds = []
         self.environment_changed = []
-        self.rerun = []
         self.run_no_tests = []
+        self.rerun = []
         self.first_result = None
         self.interrupted = False
 
@@ -105,6 +105,11 @@ class Regrtest:
         # used by --junit-xml
         self.testsuite_xml = None
 
+    def get_executed(self):
+        return (set(self.good) | set(self.bad) | set(self.skipped)
+                | set(self.resource_denieds) | set(self.environment_changed)
+                | set(self.run_no_tests))
+
     def accumulate_result(self, result):
         test_name = result.test_name
         ok = result.result
@@ -311,8 +316,6 @@ class Regrtest:
                 self.bad.remove(test_name)
 
             if ok.result == INTERRUPTED:
-                # print a newline separate from the ^C
-                print()
                 self.interrupted = True
                 break
         else:
@@ -331,11 +334,11 @@ class Regrtest:
         print("== Tests result: %s ==" % self.get_tests_result())
 
         if self.interrupted:
-            print()
-            # print a newline after ^C
             print("Test suite interrupted by signal SIGINT.")
-            executed = set(self.good) | set(self.bad) | set(self.skipped)
-            omitted = set(self.selected) - executed
+
+        omitted = set(self.selected) - self.get_executed()
+        if omitted:
+            print()
             print(count(len(omitted), "test"), "omitted:")
             printlist(omitted)