Make GAINROOT variable overridable by an environment variable.
Add support for "unsafe" tests.

They are meant to contain tests that modify the real system and can be
potentially harmful. They can be turned on by using the --unsafe
option or using the UNSAFE_TESTS environment variable.
Add documentation of "--unsafe" option to testall.
Updated documentation with regards to test naming.
Merge pull request #948 from kacfengine/contained-tests

Add support for unsafe tests in testall