Flaky: Csharp SDK Test (#1651)

Theory is that the Csharp SDK build is in a race condition with the test
system. So we'll try making tests dependent on build-sdks to see if that
solves the issue.

Closes #1639

cloudbuild.yaml

@@ -189,6 +189,7 @@ steps:
     - lint
     - ensure-build-sdk-image-base
     - htmltest-restore-cache
+    - build-sdks
   dir: "build"
   args: [ "-j", "5", "--output-sync=target", "test" ]