bpo-30523: regrtest: Add --list-cases option (#2238)

* bpo-30523: regrtest: Add --list-cases option

* bpo-30523: Enhance --list-cases

* Add get_abs_module() function, use it in list_cases()
* list_cases() now logs skipped tests into stderr

* Remove unused doctest
diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index 7f66997..bf64062 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -250,6 +250,9 @@
     group.add_argument('--list-tests', action='store_true',
                        help="only write the name of tests that will be run, "
                             "don't execute them")
+    group.add_argument('--list-cases', action='store_true',
+                       help='only write the name of test cases that will be run'
+                            ' , don\'t execute them')
     group.add_argument('-P', '--pgo', dest='pgo', action='store_true',
                        help='enable Profile Guided Optimization training')