Skip ls tests that use sparse files if these are not supported.

Some of the ls(1) tests create really large sparse files to validate
the number formatting features of ls(1). Unfortunately, those tests fail
if the underlying test file system does not support sparse files, as is the
case when /tmp is mounted on tmpfs.

Before running these tests, check if the test file system supports sparse
files by using getconf(1) and skip them if not. Note that the support for
this query was just added to getconf(1) in r304694.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D7609