fix(jest): correctly set coverageThreshold

[charlieTheBotDev]

coverageThreshold was never being set in jestConfig as it is an object

See https://jestjs.io/docs/en/configuration.html#coveragethreshold-object

Note: https://stenciljs.com/docs/testing-config doesn't mention anything about it being a special option so if it should be a string then the docs need updating instead of merging this PR.

I also tried using JSON.stringify to set it but that just made Jest fall over with:

[ ERROR ]  runJest: ● Validation Error:   Option
           "coverageThreshold" must be of type:  object  but
           instead received:  string   Example:  {
            "coverageThreshold": {  "global": {  "branches":
           50,  "functions": 100,  "lines": 100,
           "statements": 100  }  }  }
           Configuration Documentation:  https://jestjs.io/docs/configuration.html

[manucorporat]

Thanks for your PR!