Summary:
The `NoWaitForRPCOnTermination` unit test expects that connection will be closed while query processing. The test checks that raised error will have particular message `terminating connection due to administrator command`. This error message with this substring is really logged by postgres in logs, but the postgres process may not sent it to the user. It may just close the connection. As a result client may only detect general connection close error.

Solution: rework the check that connection was closed.
Jira: DB-13330

Test Plan:
Run existing unit test
```
./yb_build.sh --gtest_filter PgMiniTest.NoWaitForRPCOnTermination
```

Reviewers: pjain, jason, kramanathan

Reviewed By: kramanathan

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39005