Properly skip coreutils readlink tests

Some tests in VirtDiskVFSLocalFSTestPaths class are skipped
if we're running on OSX by doing a check using is_osx() function
from tests.utils. However, these tests actually require readlink
from coreutils, so replace OS X check by more general check
for coreutils readlink presence.

This change allows to run a complete test suite on OSes rather than
Linux and OSX, such as FreeBSD.

These tests were the only consumer of tests.utils.is_osx, but
it's kept because it could be still potentially useful.

Change-Id: I7593560b84bda3719de811fbc214b1f52e1a121f
Merge "Properly skip coreutils readlink tests"