test: enable mysql tests on Jenkins

This is the part of #134 that re-enables the tests that depend on a DB, since our Jenkins servers support it.

/to @bajtos 
Minor nick: either put each argument on it's own line, or even better - extract the first arg expression into a variable:

```js
var invokePath = require.resolve('../bin/datasource-invoke');
child = fork(invokePath, [], { silent: true });
```