Add subscription tests for disconnect & setProvider (#3190)

[cgewecke]

Description

PR targets #3329 (If you merge 3329 first, this should go into #3190 clean, but you'll have to change the target branch).

Adds a couple final tests for #3190, describing expected behavior on provider.disconnect and eth.setProvider.

The provider disconnect test reproduces the error triggered in the ganache E2E, and can be seen failing in Travis here in the first commit. When a delay added between disconnect and sever shutdown, the test succeeds.

Have opened ganache-core 545 to address their test failure - they should be able to bump to a higher patch version without problems after #3190 is published.

Type of change

• Tests

Checklist:

• I have selected the correct base branch. Based on Check for error codes on close in RequestManager (#3190) #3329
• 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 (+0.07%) to 88.116% when pulling e78bed7 on pri/isolation-tests into 46ebb24 on pri/connection-close.