[Piglit] Add module specifically for working with group strings

[Dylan Baker]

One of the most confusing things about piglit is the use of posixpath
for working with group strings. This is obviously convenient since
piglit uses '/' separated groups, but leads to problems.

The first problem is that sometimes os.path gets used when posixpath
should be used, which works on Linux but breaks on windows. The second
problem is that well meaning people see posixpath and replace it with
os.path.

By using an explicit module (even if that module largely wraps
posixpath, we gain clarity and self documentation.