Fix Flaky gRPC server down test
Summary: Rather than SIGKILLing the server process to test it going down, which breaks tests in various ways, simulate the server being down by pointing the run at an entirely different port.
Test Plan: BK + Azure
Reviewers: sashank, alangenfeld, max
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D4556