gh-110722: Make -m test -T -j use sys.monitoring (GH-111710)

Now all results from worker processes are aggregated and
displayed together as a summary at the end of a regrtest run.

The traditional trace is left in place for use with sequential
in-process test runs but now raises a warning that those
numbers are not precise.

`-T -j` requires `--with-pydebug` as it relies on `-Xpresite=`.

Lib/test/support/__init__.py

@@ -1082,18 +1082,30 @@ def check_impl_detail(**guards):
 
 def no_tracing(func):
     """Decorator to temporarily turn off tracing for the duration of a test."""
-    if not hasattr(sys, 'gettrace'):
-        return func
-    else:
+    trace_wrapper = func
+    if hasattr(sys, 'gettrace'):
         @functools.wraps(func)
-        def wrapper(*args, **kwargs):
+        def trace_wrapper(*args, **kwargs):
             original_trace = sys.gettrace()
             try:
                 sys.settrace(None)
                 return func(*args, **kwargs)
             finally:
                 sys.settrace(original_trace)
-        return wrapper
+
+    coverage_wrapper = trace_wrapper
+    if 'test.cov' in sys.modules:  # -Xpresite=test.cov used
+        cov = sys.monitoring.COVERAGE_ID
+        @functools.wraps(func)
+        def coverage_wrapper(*args, **kwargs):
+            original_events = sys.monitoring.get_events(cov)
+            try:
+                sys.monitoring.set_events(cov, 0)
+                return trace_wrapper(*args, **kwargs)
+            finally:
+                sys.monitoring.set_events(cov, original_events)
+
+    return coverage_wrapper
 
 
 def refcount_test(test):