Add coverage for duplicate file closes

[devinivy]

As of hapijs/hapi#4257 hapi no longer naturally causes a file response to be closed multiple times, however we don't want inert to rely on this assumption quite yet. So for hapi v20.1.4+ this new test should ensure we're covered. The branch in question is here:

inert/lib/file.js

Lines 248 to 252 in f932e40

	internals.close = function (response) {
	const { source } = response;
	if (source.file !== null) {

This should fix CI for #157.

[Nargonath]

Code LGTM but there are some failed tests on OSX. That's perhaps some flacky tests acting up but I'm not sure. I'll have to dig a bit more.

[devinivy]

Yeah, it's a flaky lsof test. I don't know why it's flaky only on OSX (it isn't flaky on my OSX machine), but I'd be interested to know if those machines might be running other workloads. If so, we might need to tighten the lsof checks to ensure it's looking for a specific file named "package.json" rather than any file named "package.json."

[devinivy]

@Nargonath I tracked it down and have included the changes to fix that flakiness 👍

[Nargonath]

Good job @devinivy .