Increase default timeout for running cell tests

[ceball]

Currently, once you click to run cell tests in jupyter lab, the result needs to come back within 5 seconds or it'll be cancelled.

But even if all heavy parts of a notebook are mocked out, many a notebook's celltests will not complete in 5 seconds.

We should probably up the default to 15 seconds or so.

nbcelltests/js/src/run.ts

Line 15 in fa86221

import {IRequestResult, request} from "requests-helper";

nbcelltests/js/src/run.ts

Line 35 in fa86221

request("post",

https://github.com/timkpaine/requests-helper/blob/master/src/index.ts#L21

That should be easy.

We could also consider making that configurable.

However, it might be better to focus on improving how celltests are launched: #210

[vidartf]

I think this is an artifact of the current UX. With a better UX, we could up the timeout a lot (and make it a setting), and instead ensure that the user is able to cancel the test run e.g. by pressing a button.