Remove goroutines from transportshim tests
Remove goroutines from transportshim tests

This commit fixes a ~1-in-1000 flake in transportshim's test suite by
removing the goroutine that wraps the calls to shim.Write(), as it is
not necessary to write from a separate thread. This fix was verified
by running the tests 100k times and observing zero failures.

PiperOrigin-RevId: 398570605
Change-Id: I8c089c001a511fb59439b0df31255653a58b3f7b

GoogleCloudPlatform/stet