fix: pin testcafe version to fix faulty e2e

[gweiying]

Problem

Our E2E tests are breaking as we have not pinned Testcafe's version, and there seems to be some bugs in handling asynchronicity in Testcafe's new v1.19 release from a couple of days ago.

Details
Tests are failing with the following error thrown by Testcafe's internal libraries:

internal/util.js:279
    throw new ERR_INVALID_ARG_TYPE('original', 'Function', original);
    ^
TypeError [ERR_INVALID_ARG_TYPE]: The "original" argument must be of type function. Received undefined
at promisify (internal/util.js:279:11)
    at Object.<anonymous> (/home/runner/work/GoGovSG/GoGovSG/node_modules/testcafe-safe-storage/lib/file.js:18:33)

More details will be added here after investigating the error more, but this should be a non-blocking investigation. We should pin our Testcafe version regardless as we do not want updates to the library to break our testing.

Solution

Pin Testcafe to v1.18.6

[yong-jie]

thanks for helping with this! this approach lgtm