firebase-ios-sdk

Add cross-platform support for creating Executors (#3897)

* Add Executor::CreateSerial

... a standard way to create an Executor for most cases where we don't
care about the implementation.

* Add ExecutorForTesting/AsyncQueueForTesting

Uses Executor::CreateSerial to do the heavy lifting.

* Use AsyncQueueForTesting

* Avoid direct references to ExecutorLibdispatch in prod code

* Rewrite time_testing in terms of absl::Time

* Migrate async_test_util's now to time_testing

* Fix AddressSanitizer stack-buffer-underflow error

This was reading random data off the stack until it hit a negative
integer's worth of data. The test passed because this was generating the
input and expected values, which were much longer than expected given
the input.