Add patcher for disabling quickinstaller snappshotting in tests.

Quickinstaller normally makes a complete Generic Setup (GS) snapshot before and after installing each GS profile, in order to be able to uninstall the profile afterwards.

In tests we usually don't need this feature and want to disable it to speed up tests.

The ``ftw.testing.quickinstaller`` module provides a patcher for replacing the quickinstaller event handlers to skip creating snapshots.
Usually we want to do this early (when loading ``testing.py``), so that all the tests are speeding up.
However, some tests which involve quickinstaller rely on having the snapshots made (see previous section about uninstall tests).
Therefore the snapshot patcher object provides context managers for temporarily enabling / disabling the snapshot feature.

**Usage:**

Disable snapshots early, so that everything is fast. Usually this is
done in the ``testing.py`` in module scope, so that it happens already
when the testrunner imports the tests:

```python
from ftw.testing.quickinstaller import snapshots
from plone.app.testing import PloneSandboxLayer

snapshots.disable()

class MyPackageLayer(PloneSandboxLayer):
```

When testing quickinstaller snapshot related things, such as uninstalling, the snapshots can be re-enabled for a context manager or in general:
```python
from ftw.testing.quickinstaller import snapshots

snapshots.disable()
# snapshotting is now disabled

with snapshots.enabled():
    # snapshotting is enabled only within this block

snapshots.enable()
# snapshotting is now enabled

with snapshots.disabled():
    # snapshotting is disabled only within this block
```