(20967) Clear yamldir in configuration acceptance test.

Because yamldir subdirectories are not managed outside of
indirectory/yaml, service user changes can leave puppet unable to read
from them (#21145)

This change clears the yamldir in the puppet_manages_own_configuration_in_robust_manner
acceptance test to ensure that yamldir and all subdirs are rebuilt with
the new service user.
(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.
Merge pull request #1698 from jpartlow/issue/master/yaml-dir-blocking-puppet-rundir-fix

(20967) Clear yamldir in configuration acceptance test.