Bump up default timeout for waiting for gRPC server

Summary: 3 seconds is quite aggressive for a process that can involve loading user code (and is manifesting as such in flaky tests on Windows). Bump up to 15 seconds.

Test Plan: BK + Azure

Reviewers: alangenfeld, max, sashank

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D4906