mutter-test: move Test class into package

I decided to split mutter-test script into more files
1) it's better 2) author intended do it later according the comments :)

I created a package muttertest where will be placed classes and helper
functions that do not need be neccessarily (or logically) in
mutter-test.

This patches is the first in series that do abovementioned stuff and
moves base class Test into the package.
mutter-test: move and rename run() function

This function has the same name as Test.run() method and this can be
confusing. Rename it to sys_run and move it into helpers.py source
mutter-test: move exceptions to standalone file

continue packaging