Avoid accidentally killing the teardown thread

[headius]

If the main thread performing the teardown appears earlier in this map, we may skip the teardown of remaining threads. Instead, we avoid the kill+join dance for the current thread to ensure all other threads get a chance to terminate.

[headius]

Relates to #7349 and ruby/timeout#21.

[headius]

This is the primary cause of failures on master in the ScriptingContainerTest.testTimeoutCleanup due to the new timeout implementation using a background thread for all timeouts, which is left dangling until process termination. When the teardown thread is prematurely killed, this dangling thread does not get cleaned up and the test fails.

The timeout library should still be making a best effort to shut down its thread gracefully (rather than waiting for it to be forcibly killed) but this change should get the test green on master.