TestConcurrentRangeQueries: log query with error

We've seen some timeouts in CI, and wanted to know what queries are
involved.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/promql_test.go

@@ -87,6 +87,7 @@ func TestConcurrentRangeQueries(t *testing.T) {
 			}
 			res := qry.Exec(context.Background())
 			if res.Err != nil {
+				t.Logf("Query: %q, steps: %d, result: %s", c.expr, c.steps, res.Err)
 				return res.Err
 			}
 			qry.Close()