Issue #18492: Allow all resources when tests are not run by regrtest.py.

This changeset also includes cleanup allowed by this behavior change.
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 87e8816..0902278 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -2271,7 +2271,7 @@
                                   "operation raises different flags depending on flags set: " +
                                   "expected %s, got %s" % (expected_flags, new_flags))
 
-def test_main(arith=False, verbose=None, todo_tests=None, debug=None):
+def test_main(arith=None, verbose=None, todo_tests=None, debug=None):
     """ Execute the tests.
 
     Runs all arithmetic tests if arith is True or if the "decimal" resource
@@ -2280,7 +2280,7 @@
 
     init()
     global TEST_ALL, DEBUG
-    TEST_ALL = arith or is_resource_enabled('decimal')
+    TEST_ALL = arith if arith is not None else is_resource_enabled('decimal')
     DEBUG = debug
 
     if todo_tests is None: