CMS 5: Fix broken JavaScript tests

[GuySartorelli]

In some modules, javascript tests are broken after #1312
For those modules, I removed the yarn test command from yarn build so that the resources can be built.

The tests must be fixed and passing, then yarn test must be added back into the yarn build command.
Use #1389 as an example of some changes you may need and acceptable workarounds (e.g. resorting to react 16 for tests, for now)

Acceptance Criteria

• yarn test runs without failures on all modules
• yarn test is added back into the yarn build command

PRs

• FIX Remove redundant memoisation #1456
• FIX Get JS tests passing again silverstripe-asset-admin#1330
• MNT Get JS tests passing again silverstripe-campaign-admin#256
• MNT Get JS tests passing again silverstripe-cms#2834
• MNT Get JS tests passing again silverstripe-session-manager#128
• MNT Get JS tests passing again silverstripe-versioned-admin#274

[emteknetnz]

Linked PRs have been merged