test: 100% coverage for `tslib.ts` (error case)

[agilgur5]

trafficstars

Summary

Get to 100% coverage of tslib.ts by adding a check for error case

• Follow-up to #321

Details

• use Jest's module mocks to test the error case of tslib.ts when tslib fails to import
  • this took a bit of work to figure out bc, per the in-line comments, the ordering and module subpath were both required
    • it was pretty confusing for a while until I realized what might be going wrong