roachtest: fix flakes in npgsql test

**Describe the problem**

Many tests in the npgsql nightly roachtest are ignored, since they all have a common way of flaking. An example is:
```
Failed Cursor_move_RecordsAffected [2 ms]
Error Message:
Npgsql.NpgsqlException : Received backend message ReadyForQuery while expecting ParseCompleteMessage. Please file a bug.
Stack Trace:
at Npgsql.Util.Statics.ThrowIfMsgWrongType[T](IBackendMessage msg, NpgsqlConnector connector) in /mnt/data1/npgsql/src/Npgsql/Util/PGUtil.cs:line 49
at Npgsql.Util.Statics.Expect[T](IBackendMessage msg, NpgsqlConnector connector) in /mnt/data1/npgsql/src/Npgsql/Util/PGUtil.cs:line 32
at Npgsql.NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken) in /mnt/data1/npgsql/src/Npgsql/NpgsqlDataReader.cs:line 451
at Npgsql.NpgsqlDataReader.NextResult(Boolean async, Boolean isConsuming, CancellationToken cancellationToken) in /mnt/data1/npgsql/src/Npgsql/NpgsqlDataReader.cs:line 608
at Npgsql.NpgsqlDataReader.NextResult() in /mnt/data1/npgsql/src/Npgsql/NpgsqlDataReader.cs:line 342
at Npgsql.NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken) in /mnt/data1/npgsql/src/Npgsql/NpgsqlCommand.cs:line 1466
at Npgsql.NpgsqlCommand.ExecuteReader(CommandBehavior behavior, Boolean async, CancellationToken cancellationToken) in /mnt/data1/npgsql/src/Npgsql/NpgsqlCommand.cs:line 1542
at Npgsql.NpgsqlCommand.ExecuteNonQuery(Boolean async, CancellationToken cancellationToken) in /mnt/data1/npgsql/src/Npgsql/NpgsqlCommand.cs:line 1177
at Npgsql.NpgsqlCommand.ExecuteNonQuery() in /mnt/data1/npgsql/src/Npgsql/NpgsqlCommand.cs:line 1159
at Npgsql.Tests.CommandTests.Cursor_move_RecordsAffected() in /mnt/data1/npgsql/test/Npgsql.Tests/CommandTests.cs:line 485
at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.GetResult()
at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(Func`1 invoke)
at NUnit.Framework.Internal.Commands.TestMethodCommand.RunTestMethod(TestExecutionContext context)
at NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext context)
at NUnit.Framework.Internal.Commands.TimeoutCommand.<>c__DisplayClass5_0.b__0()
at System.Threading.Tasks.Task`1.InnerInvoke()
at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
at NUnit.Framework.Internal.Commands.TimeoutCommand.Execute(TestExecutionContext context)
```

Determine what causes that, and stop ignoring these upstream tests in this list: https://github.com/cockroachdb/cockroach/blob/a3a389e464defd3d301a0b080435cf33502a02d2/pkg/cmd/roachtest/tests/npgsql_blocklist.go#L747

Jira issue: CRDB-30454

Epic CRDB-19022