Add and use method to test a package's exported values

[pjlee11]

Part 1 of 2 for #156

Follows up on #224

Adds a test helper method that takes an actual import and validates it has all the exported values it is expected to have. This can then be used to simplifying testing the other utility packages exports as well as psammead-test-helpers

Has error handling logic for both new files (EG: font.js) and for exported files within those files when:

• The actual import has additional exports when compared to the expected values. This is needed so that we do not forget to change the expectations when adding a new export.
• The expected value has additional exports as compared to the actual exports. This is needed in the case of a breaking change when an export is to be removed. This is very useful at catching regressions in the utility packages.

• I have assigned myself to this PR and the corresponding issues
• Tests added for new features
• Test engineer approval

Testing this involves ensuring no regression in the psammead-test-helpers package, which can be verified via running npm run build & npm run test. Also the psammead-test-helpers package test coverage should increase from 0% to 51.28%

[pjlee11]

snyk stalling on jobs so re-committing ...

[pjlee11]

Confirmed that synk is hanging as the webhooks are looking at the wrong github repo

[jamesbrumpton]

LGTM 👍