GH-35328: [Go][FlightSQL] Fix flaky test for FlightSql driver (#38044)

### Rationale for this change
Fixing a flaky test that is *very* difficult to reproduce.

### What changes are included in this PR?
Adding an explicit call to drain the remaining batches from the parameter batch reader in the `DoPutPreparedStatementQuery` of the `MockServer` in the tests. This ensures that the gRPC connection doesn't close on us in between the client writing the schema message and writing the record batch message producing an `io.EOF` error. This is an extremely rare occurence based on goroutine scheduling due to the client and mockserver both running in the same process for the tests, in local testing i was only able to get it to happen 1 - 4 times per 5000 runs of the test.

### Are there any user-facing changes?
No

* Closes: #35328

Authored-by: Matt Topol <[email protected]>
Signed-off-by: Matt Topol <[email protected]>

go/arrow/flight/flightsql/driver/driver_test.go

@@ -812,6 +812,17 @@ func (s *MockServer) DoPutPreparedStatementQuery(ctx context.Context, qry flight
 		return fmt.Errorf("parameter schema: %w", arrow.ErrInvalid)
 	}
 
+	// GH-35328: it's rare, but this function can complete execution and return
+	// closing the reader *after* the schema is written but *before* the parameter batch
+	// is written (race condition based on goroutine scheduling). In that situation,
+	// the client call to Write the parameter record batch will return an io.EOF because
+	// this end of the connection will have closed before it attempted to send the batch.
+	// This created a flaky test situation that was difficult to reproduce (1-4 failures
+	// in 5000 runs). We can avoid this flakiness by simply *explicitly* draining the
+	// record batch messages from the reader before returning.
+	for r.Next() {
+	}
+
 	return nil
 }