gh-109700: Improve stress tests for interpreter creation

[serhiy-storchaka]

• Ensure that destructors are called in the test that created interpreters, not after finishing it.
• Try to create interpreters simultaneously in test_create_many_threaded.
• Mark test_create_many_threaded that requires over 6GB of memory as bigmemtest.

• Issue: test_interpreters: test_create_many_threaded() failed on FreeBSD: log: RuntimeError: interpreter creation failed #109700

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @serhiy-storchaka for commit 6aecbf9 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[serhiy-storchaka]

While leaks are not yet fixed, we want to minimize effect of these tests on other tests.