tests: fix problems in the m_rootgnutar test

tests: fix problems in the m_rootgnutar test

The mkgnutar.pl file only works if the developer had a specific
username and uid.  In addition, if it is used, the round-trip from tar
to an ext4 file system and back to tar isn't properly tested.  So only
use mkgnutar.pl if the system doesn't have GNU TAR.

In addition, make sure all of the temp files created by the test are
deleted when the test is completed.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>