Clean up duplicated port management code (#2749)
Clean up duplicated port management code (#2749)

The VA previously had a lot of repetitive code of the form:
```
	hs := httpSrv(t, expectedToken)
	defer hs.Close()
	port, err := getPort(hs)
	test.AssertNotError(t, err, "failed to get test server port")
	va, _, log := setup()
	va.httpPort = port
```

This PR moves the port management to the `setup()`  func and updates the tests accordingly.

Resolves https://github.com/letsencrypt/boulder/issues/2740
ibukanov/boulder,ibukanov/boulder,jsha/boulder,lucasallan/boulder,jfrazelle/boulder,jfrazelle/boulder,bmw/boulder,jsha/boulder,lucasallan/boulder,jcjones/boulder,lucasallan/boulder,jcjones/boulder,jcjones/boulder,bmw/boulder,letsencrypt/boulder,jfrazelle/boulder,jcjones/boulder,letsencrypt/boulder,jsha/boulder,ibukanov/boulder,jfrazelle/boulder,letsencrypt/boulder,bmw/boulder