update CP2K easyblock

builds on top of @wpoely86's PR #323, fixes bug in definition of `test_core_cnt` (which is not a real constant since it's not defined out of the class) and includes minor style fixes

tested with https://github.com/hpcugent/easybuild-easyconfigs/pull/572 and https://github.com/hpcugent/easybuild-easyconfigs/pull/601