Fix tests when tmp files have attributes.

SELinux systems may have the security.selinux attribute defined for
temporary files upon creation. This can cause problems for the tests,
which assumed that new files have no attributes. Similar problems can
arise in other cases where temporary files created by python start out
with attributes.

To work around this, a comma-separated list of attributes to ignore can
be specified in the environment variable "TEST_IGNORE_XATTRS". Any
attributes specified in this way will be filtered out by tests.