Remove unittests from core

[thekid]

This pull request implements the third part of xp-framework/rfc#293 and extracts the unittest package from core. The core tests are run via a bundled release of the unittest library (necessary not to produce recursive dependencies!)

The unittest package contents have been split into:

• https://github.com/xp-framework/unittest
• https://github.com/xp-framework/mocks
• https://github.com/xp-framework/webtest

[thekid]

Libraries will have to add a dependency to their composer.json when this is implemented.

"require" : {
  "xp-framework/core": "^6.3",
  "xp-framework/unittest": "^6.5"   //<--this
}

[thekid]

Postponed until 7.0.0-RELEASE, this would break too many libraries using ~6.0 as dependency (the default before the ^-operator was introduced).

From 6.5.0 on, the extracted libraries will work as overlays (see #96): This way, libraries can add a dependency to them (and also benefit from new features), but if they don't, they'll just use the bundled version.

[thekid]

Implemented in https://github.com/thekid/core/tree/seven