Add npm alias scripts for generating html cov report locally

[cgewecke]

Description

Per comment 3325, PR adds some npm alias scripts that let you generate an html coverage report locally.

npm run test:cov will run the unit and client tests, generating an aggregate coverage report that can be viewed in a browser by opening web3.js/coverage/index.html.

Have left out test:e2e:ens because it installs stuff in the project root - e.g it's designed to run in CI rather than locally.

You can also combine commands to generate coverage more selectively if you're trying to look at the result of specific tests. e.g This will generate html coverage for geth:insta tests only:

npm run cov:clean 
npm run test:e2e:geth:insta 
npm run cov:html

Example:

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success and extended the tests if necessary.
• I ran npm run build-all and tested the resulting file/'s from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.

[coveralls]

Coverage remained the same at 85.435% when pulling 0a10f77 on test/local-coverage into f5f7a42 on 1.x.

[nivida]

LGTM! Well described PR and a smart solution great!

[nivida]

@cgewecke I will extend the checklist in the PR template with the following addition:

- [ ] I have executed ``npm run test:cov`` and my test cases do cover all lines and branches.