test:Ensure worker thread waits for test assertion

[marcotc]

While testing Datadog::Workers::Async::Thread#running?, we could encounter a race condition where we start a background task but that task finishes before we can check the result of #running?:

dd-trace-rb/spec/ddtrace/workers/async_spec.rb

Lines 266 to 274 in 6e326cc

	context 'when started' do
	before do
	worker.perform
	try_wait_until { worker.running? }
	end
	after { worker.terminate }
	it { is_expected.to be true }
	end

This happens because the task we schedule to run is empty, finishing very quickly.

This PR changes the background task to wait for some time, so we can assert on it. The thread won't hold our test suite back because we call worker#terminal, which forcefully terminates the thread.

[ericmustin]

Good catch, 👍 lgtm