Differentiate starting the daemons or not.

Unit tests also need the configuration files.
Also differentiate starting the daemons or not
Fix start daemons references
Allow passing the salt checkout directory in case it's not installed
Move `SaltReturnAssertsMixIn` from salt's source code
Migrate Salt's `TestCase` classed into `salttesting.cases`
Migrate `run_tests` from salt's source code

This will only show an error to the user and the example usage to use
`salt-runtests`
Salt modules need to be late imported

Since salt might not be importable and the `PYTHONPATH` environment
variable is not set early enough.
We need an absolute path
Make the salt integration test files directory available as a global variable
Let's keep the original test name on test import errors
Remove unused code
Let's keep the original test name on test import errors