Backport to 16.4 the fix for test failure due to "not-existing.com" being registered

Gitaly used to have a test that verifies Gitaly handles a non-existing domain correctly. Unfortunately, someone registered not-existing.com domain a while ago. It broke the test.

Error:      	Expect "send ping: failed to connect to `host=not-existing.com user=postgres database=postgres`: dial error (dial tcp 38.242.148.114:5432: connect: connection refused)" to match "send ping: failed to connect to `host=not\-existing.com user=.* database=.*`: hostname resolving error"

We detected that issue and opened a fix (!6488 (merged)), which is to remove that test, for it. This MR is to back-port the fix to 16.4.