When running tests I got this error:
```
1) multilayer error cases bogus sql raises 400 status code:
TypeError: Cannot read property 'listen' of undefined
at Function.assert.response (test/support/assert.js:93:26)
at Function.requestLayergroup (test/acceptance/ported/support/test_client.js:79:20)
at next (node_modules/step/lib/step.js:51:23)
at Step (node_modules/step/lib/step.js:122:3)
at Object.createLayergroup (test/acceptance/ported/support/test_client.js:75:5)
at Context.<anonymous> (test/acceptance/ported/multilayer_error_cases.js:304:20)
```
The problem is that `server` is declared but its initialization
may depend on the order of execution of suites, which is basically
that of the filesystem/checkouts.
That is fixed by returning `getServer()`, which seems to be the
original intent: return a singleton of `CartodbServer` properly
initialized in case it is not overriden through options.
