Close all connections in all tests, also change some tests to be working stand-alone

[hasezoey]

Summary

This PR updates some tests / test files to close all connections that were opened, also changes some test files to be working stand-alone

though i am not really proud of some of the connection caching done with connectionsToClose

some more detailed notes:

• all test files except connection.test.js and index.test.js have been updated with connectionsToClose or a simple mongoose.disconnect instead of not closing the connections
• some spaces have been added between some tests
• many test/helpers test files have been updated to work when tested stand-alone (only testing a single test file)
• change some import order in test/helpers to be consistent across test/helpers test files
• some tests have been updated to use the existing connection instead of creating a new one (when it was not about using a new connection)

these changes are required for a follow-up PR i am doing to integrate MMS to the deno tests too (instead of manually downloading and starting mongodb), because otherwise it would fail with the following when the single-instance is stopped:

error: Uncaught Error: connect ECONNREFUSED 127.0.0.1:36683 - Local (undefined:undefined)
    Error.captureStackTrace(err);
          ^
    at __node_internal_captureLargerStackTrace (https://deno.land/std@0.171.0/node/internal/errors.ts:113:11)
    at __node_internal_exceptionWithHostPort (https://deno.land/std@0.171.0/node/internal/errors.ts:295:12)
    at TCPConnectWrap._afterConnect [as oncomplete] (https://deno.land/std@0.171.0/node/net.ts:369:16)
    at TCP.afterConnect (https://deno.land/std@0.171.0/node/internal_binding/connection_wrap.ts:71:11)
    at https://deno.land/std@0.171.0/node/internal_binding/tcp_wrap.ts:375:16
error Command failed with exit code 1.

[vkarpov15]

Instead of close() in an after, why not just declare let db = null at top-level describe block, and then afterEach(() => db && db.close()) ? That way tests don't need an extra connectionsToClose.push() call, just replace const db = mongoose.createConnection() with db = mongoose.createConnection() ? That would avoid the need for extra boilerplate in tests.

[hasezoey]

i did this to not modify the test too much, also some tests use multiple connections

[vkarpov15]

Let's not add extra done() usage, we should avoid using done(). return db.close() should be fine because db.close() returns promise if no callback specified.

[vkarpov15]

Same, avoid adding done.

[vkarpov15]

Let's do let db = null; instead and close() in an afterEach().

[vkarpov15]

I'm just going to merge this one and make suggested changes in separate PR. Thanks 👍

[hasezoey]

I'm just going to merge this one and make suggested changes in separate PR. Thanks +1

also good, just wanted to know before how to handle multiple connections in a test with the suggested way, because a single let db = connection would not be sufficient and multiple let db1 = connection; let db2 = connection; would with even more connections become redundant