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/cov.py

@@ -0,0 +1,48 @@
+"""A minimal hook for gathering line coverage of the standard library.
+
+Designed to be used with -Xpresite= which means:
+* it installs itself on import
+* it's not imported as `__main__` so can't use the ifmain idiom
+* it can't import anything besides `sys` to avoid tainting gathered coverage
+* filenames are not normalized
+
+To get gathered coverage back, look for 'test.cov' in `sys.modules`
+instead of importing directly. That way you can determine if the module
+was already in use.
+
+If you need to disable the hook, call the `disable()` function.
+"""
+
+import sys
+
+mon = sys.monitoring
+
+FileName = str
+LineNo = int
+Location = tuple[FileName, LineNo]
+
+coverage: set[Location] = set()
+
+
+# `types` and `typing` aren't imported to avoid invalid coverage
+def add_line(
+    code: "types.CodeType",
+    lineno: int,
+) -> "typing.Literal[sys.monitoring.DISABLE]":
+    coverage.add((code.co_filename, lineno))
+    return mon.DISABLE
+
+
+def enable():
+    mon.use_tool_id(mon.COVERAGE_ID, "regrtest coverage")
+    mon.register_callback(mon.COVERAGE_ID, mon.events.LINE, add_line)
+    mon.set_events(mon.COVERAGE_ID, mon.events.LINE)
+
+
+def disable():
+    mon.set_events(mon.COVERAGE_ID, 0)
+    mon.register_callback(mon.COVERAGE_ID, mon.events.LINE, None)
+    mon.free_tool_id(mon.COVERAGE_ID)
+
+
+enable()