bump timeout on overlapping write requests test

At 50ms, we see sporadic failures in CI. So much for "this should
only need a couple milliseconds". Bumped timeout to avoid that.

The challenge here is that we have some tests which *want* to hit
the timeout to confirm that we aren't allowing things we shouldn't.
But we don't want the test to hang forever. But we want to be sure
it is actually stuck and not just being slow...

(cherry picked from commit f1a68f8)

querycontext/query_context_test.go

@@ -400,7 +400,7 @@ func testOverlappingWriteRequests(t *testing.T, write overlappingWriteReqs) erro
 			}
 			t.Fatalf("running reqs: %v", err)
 		}
-	case <-time.After(50 * time.Millisecond):
+	case <-time.After(250 * time.Millisecond):
 		if !write.shouldError {
 			// Only log this if we didn't expect an error.
 			t.Logf("write requests (%d):", len(write.reqs))