close koa servers when if tests finish

[Ohadbasan]

This PR fixes the problem described at
#437
supertest did not shut down koa.js servers when tests finish running.

[coveralls]

Pull Request Test Coverage Report for Build 345

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 96.861%

---

Totals
Change from base Build 330:	-0.4%
Covered Lines:	136
Relevant Lines:	140

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 345

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 96.861%

---

Totals
Change from base Build 330:	-0.4%
Covered Lines:	136
Relevant Lines:	140

---

💛 - Coveralls

[Ohadbasan]

I see that the node 10 tests fail
not sure what causes the behaviour change.
I can investigate that.
if (!addr || (addr.address === '::' && !process.version.startsWith('v10'))) { this._server = app.listen(0); }

this will 'solve' the problem but I guess we don't want that