add `compare_versions()` to test_functions
add `is_kernel_vulnerable()` to test case 564
use `is_kernel_vulnerable()` function
log $CVMFS_TEST_SERVER_CACHE in test case 564 (debugging)
disentangle logging and checking for `cvmfs_server transaction`
adapt return values
Merge pull request #574 from reneme/fix-test_564

FIX: Test 564 is Hard to Debug