Two changes to the `jest` tests:

- We are currently running the tests directly from `.ts` source files
  using `ts-jest`. Disable type checking (by setting `isolatedModules:
  true`) to improve the speed. This brings down running 4 test suites
  from an arbitrary service (`aws-servicecatalog`) down from `79s` to
  `19s` on my machine.
- Change the "coverage threshold" ignore files a little to get rid of
  the "Note: The code generator has deoptimised the styling" errors
  produced by jest when it wants to read the coverage of the generated
  source files.