[Feature] Install cnfs into system-wide directory

[kosstennbl]

Currently, cnf_install task creates "cnfs" folder in the current directory and uses it for storing data used by other tasks/tests.

Problem is that this approach locks the user to the directory from where he runs testsuite, if directory is changed between installation and running tests - testsuite wouldn't work correctly.

Idea is to fix this behavior by making cnfs directory user-wide, placed in a single defined space "i.e. ~/.cnf-testsuite directory". That would allow for more robust and predictable performance of cnf-testsuite, and would allow for better implementation of some other issues (i.e. #2095).