WASM: Add an internal method to pump the threadpool (#38690)

WASM: Add an internal method to pump the threadpool (#38690)

This can by used by the xharness xunit runner to make sure async tasks are executed.

We also need to tweak how `ThreadPoolTaskScheduler` handles `TaskCreationOptions.LongRunning` since the usual mode of starting a new thread doesn't work, instead we treat it like the option wasn't set and queue the task on the threadpool.