chore: We can now upgrade to mocha@5 🌮

## Symptoms

Upgrading to `mocha@5` now causes my tests to hang without any context. [See this Travis build as an example](https://travis-ci.org/mongodb-js/index-model/builds/451375910)

## Resolution

We were *not* explicitly closing driver connections after tests that needed it finished which kept the driver's underlying sockets to remain open. These `after(client.close)` blocks have been added in this PR.

## Root Cause

`mocha@4` introduced a reenforcement for better testing and quality with the removal of mocha killing its process when it *thinks* the tests complete. [See release notes](https://boneskull.com/mocha-v4-nears-release/#mochawontforceexit)

For us, this is a huge win. `mocha` has always included its own process management which allowed us to play fast and loose releasing fds/sockets/timeouts/etc properly.

## Diagnosis

Googling lead me to [this mocha issue](mochajs/mocha#3044) where the maintainer provides several solutions to debug this and make your tests even *more* correct. Here's what worked for me:

```bash
npm i -g wtfnode;
wtfnode ./node_modules/.bin/_mocha test/data-service.test.js;
# Wait for hang to appear
# Hit ctrl+c which produces the output below
```

```
  34 passing (1s)

^C[WTF Node?] open handles:
- File descriptors: (note: stdio always exists)
  - fd 1 (tty) (stdio)
  - fd 2 (tty) (stdio)
- Sockets:
  - 127.0.0.1:57887 -> 127.0.0.1:27018
  - 127.0.0.1:57888 -> 127.0.0.1:27018
  - 127.0.0.1:57890 -> 127.0.0.1:27018
  - 127.0.0.1:57891 -> 127.0.0.1:27018
- Timers:
  - (5000 ~ 5 s) (anonymous) @ /Users/lucas/data-service/node_modules/mongodb-core/lib/topologies/server.js:373
```

package.json

@@ -14,9 +14,9 @@
     "mongodb-js"
   ],
   "scripts": {
-    "pretest": "mongodb-runner start --port=27018",
-    "test": "mocha",
-    "posttest": "mongodb-runner stop --port 27018",
+    "pretest": "wtfnode ./node_modules/.bin/mongodb-runner start --port=27018",
+    "test": "wtfnode ./node_modules/.bin/_mocha",
+    "posttest": "wtfnode ./node_modules/.bin/mongodb-runner stop --port 27018",
     "check": "mongodb-js-precommit"
   },
   "precommit": [
@@ -52,7 +52,7 @@
   "devDependencies": {
     "chai": "^3.4.1",
     "eslint-config-mongodb-js": "^3.0.1",
-    "mocha": "^3.1.2",
+    "mocha": "^5.2.0",
     "mock-require": "^2.0.1",
     "mongodb-js-precommit": "^1.1.0",
     "mongodb-runner": "^4.6.0",

test/data-service.test.js

@@ -15,6 +15,9 @@ describe('DataService', function() {
   before(function(done) {
     service.connect(done);
   });
+  after(function(done) {
+    service.disconnect(done);
+  });
 
   describe('#deleteOne', function() {
     it('deletes the document from the collection', function(done) {

test/instance-detail-helper.test.js

@@ -8,6 +8,9 @@ describe('mongodb-data-service#instance', function() {
   describe('local', function() {
     let client;
     let db;
+    after(function(done) {
+      client.close(done);
+    });
     it('should connect to `localhost:27018`', function(done) {
       const model = Connection.from('mongodb://localhost:27018/data-service');
       connect(

test/native-client.test.js

@@ -22,6 +22,10 @@ describe('NativeClient', function() {
     client.connect(callback);
   });
 
+  after(function(done) {
+    client.disconnect(done);
+  });
+
   describe('#connect', function() {
     context('when mocking connection-model', function() {
       /*