(20967) Do not run on solaris and reclear yamldir in teardown.

Cannot roundtrip users on Solaris because of #21148, so can't run the
puppet_manages_own_configuration_in_robust_manner on Solaris.

Also need to clear yamldir again in teardown since we are resetting
user/group id to original uid/gid and yaml's unmanaged subdirs will be
owned by a previous uid/gid.