Bump grpcio-testing from 1.49.1 to 1.50.0 in /test/environment/grpc

Bumps [grpcio-testing](https://grpc.io) from 1.49.1 to 1.50.0.

---
updated-dependencies:
- dependency-name: grpcio-testing
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

test/environment/grpc/requirements.txt

@@ -2,4 +2,4 @@ grpcio-tools==1.49.1
 grpcio==1.50.0
 grpcio-reflection==1.49.1
 grpcio-health-checking==1.49.1
-grpcio-testing==1.49.1
+grpcio-testing==1.50.0