Fixing invalid assumption on group name in tests

* fixed invalid assumption in file module tests that all linux platforms have
groups with the same name as users (joe:joe etc.). But for
example ArchLinux has only joe:users.

* file module tests now loads user's canonical group name from pwd and
grp python module instead.
Merge pull request #4799 from aleszoulek/fix_test_group_name

Fixing invalid assumption on group name in tests