test framework: set default concurrency, and make concurrency an explicit option in integration test framework

Title: test framework: set default concurrency, and make concurrency an explicit option in integration test framework

Description: Envoy by default starts a worker for every hardware thread, https://blog.envoyproxy.io/envoy-threading-model-a8d44b922310

for tests run on a beefy machine, this usually means either 64/72 or even 128 workers are started in an integration test.

I am not sure so many workers are desired, which may even cause noise in test. I think we should make it an explicit option to let each integration test to pick how many workers are started, and set the default to 1 or 2 if multiple workers are not desired for the test.