[es/healthcheck] ensure that healthcheck stops when server is stopped

[spalger]

While trying to write tests for #13079 I discovered that the healthcheck continues to run even after the server is closed. This fixed that by listening for the onPreStop event on the Hapi server, includes tests.

[jbudz]

Chrome 59.0.3071 (Linux 0.0.0)  "after each" hook FAILED
	Error: Uncaught TypeError: Cannot read property 'key' of undefined (http://localhost:5610/bundles/tests.bundle.js?shards=4&shard_num=1:85272)
Chrome 59.0.3071 (Linux 0.0.0): Executed 835 of 838 (1 FAILED) (0 secs / 3.468 secs)
Chrome 59.0.3071 (Linux 0.0.0)  "after each" hook FAILED
	TypeError: Cannot read property '$injector' of null
	    at Context.<anonymous> (http://localhost:5610/bundles/tests.bundle.js?shards=4&shard_num=1:87254:32)
Chrome 59.0.3071 (Linux 0.0.0): Executed 836 of 838 (2 FAILED) (0 secs / 3.474 secs)
Chrome 59.0.3071 (Linux 0.0.0)  "after each" hook for "returns a function" FAILED
	TypeError: Cannot read property '$injector' of null
	    at Context.<anonymous> (http://localhost:5610/bundles/tests.bundle.js?shards=4&shard_num=1:87254:32)

[spalger]

@jbudz merging master fixed it

[tylersmalley]

@spalger, is this good to merge?

[spalger]

5.5: #13403
5.6: #13402
6.0: b3d8523
6.1/6.x: f66daee