Add diff link for tests runned as php scripts

[danielkurecka]

This will add a link <Click to see difference> when test fails and there is line like diff 'path/to/actual' 'path/to/expected'. The link will open a diff viewer.

It uses global filters, so it will work with tests runned as regular php scripts, but it won't effect output from Nette Tester Run Configuration where such a link is already present.