Add a decorator to run a test in an isolated subprocess

Feature or enhancement

Some tests need to run in a fresh interpreter, isolated from the rest of the test run: they mutate process-global or interpreter state that would leak into (or be polluted by) other tests, depend on a clean interpreter startup, or may crash the interpreter outright. Today this has to be done ad hoc per test (spawning a child via script_helper, hand-rolling the result reporting), which is verbose and inconsistent.

I propose a decorator that runs a test in a dedicated subprocess and reports the result transparently, so the test reads like any other:

class MyTest(unittest.TestCase):
    @isolated()
    def test_global_state(self):
        ...

It would start in test.support and could later move to unittest.

Proposed behaviour

• @isolated() decorates a test method (only that method runs in a subprocess) or a whole TestCase subclass (the whole class runs in a single subprocess, so setUpClass()/setUpModule() run once there).
• Failures, errors and skips — including those of individual subTest()s — are reported for the corresponding test, and show the original subprocess traceback.
• A setUpClass()/setUpModule() failure or skip in the subprocess is reported for the whole class.
• The subprocess inherits the parent's regrtest configuration (enabled resources -u, memory limit -M, verbosity -v, failfast), so requires_resource(), requires(), bigmemtest() and similar behave the same in both processes.
• A running_isolated flag is true while running in the subprocess, so setUp()/tearDown() and the class- and module-level fixtures can choose what to run there.

Linked PRs

Linked PRs

• gh-152551
• gh-152565
• gh-152570
• gh-152639
• gh-154780
• gh-154783
• gh-154784
• gh-155162
• gh-155163
• gh-155167
• gh-155168
• gh-155169
• gh-155230
• gh-155231
• gh-155238

Bắt đầu với entry point test.support và phần xử lý subprocess hiện có trong script_helper. Xem xét các pull request được liên kết cùng với hành vi decorator được đề xuất, bao gồm việc sử dụng với method và TestCase, báo cáo kết quả, cấu hình regrtest được kế thừa và running_isolated. Hoàn tất khi các failure, skip, fixture, subTests và traceback của subprocess được chỉ định được báo cáo một cách minh bạch.