Add tests for core-subscriptions edge cases

[cgewecke]

Description

This PR targets #3190 (Provider Improvements)

Adds a few tests for edge cases in web3-core-subscriptions. Found two tiny bugs - have changed the code to fix them.

• web3.eth.subscribe('newBlockHeaders', 5), which fails in _invalidParams, gave a message like: "invalid params for "5". Expected 1, Got 1."

• web3.eth.subscribe('does-not-exist') resulted in a syntax error. It should just log a warning to std.out.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Unit tests

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 increased (+0.5%) to 85.936% when pulling 9038a7a on pri/more-ganache-tests into a46dbcd on provider-related-improvements.