Reduce too many operations mock server error

[alinaliBQ]

• Confirmed that remote tests that do not require a mock server do not run into the error GRPC_CALL_ERROR_TOO_MANY_OPERATIONS. So this error is caused by the mock server alone.
• I checked other Arrow code and Wait() call seems indeed what the codebase was missing. Wait call is meant to wait until the server fully shuts down, as Shutdown call only sends the request to shut down the server. Adding server_->Wait() reduces frequency of the error. and brings down the error from occurring around 10 iterations to 40ish iterations. The solution is not perfect, but seems that's what we can do for the time being.

Example:

arrow/cpp/src/arrow/flight/test_definitions.cc

Lines 119 to 120 in a625340

	ASSERT_OK(server->Shutdown());
	ASSERT_OK(server->Wait());

[alinaliBQ]

Closing PR as we will include this change inside #128