(#19760) Use combine when running pkginfo

pkginfo was previously executed with combine => false so stderr was
discarded. This way puppet was unable to detect any errors the pkginfo
might print, especially the message `ERROR: information for
"some_package" was not found`.

As a result the provider failed every time it tries to check the current
state of an absent package.
(#19760) Use pkginfo method instead of execute

The pkginfo method is automatically created for us and provides a nice
wrapper around execute so prefer that one over building your own execute
method invocation. Please not that the buildin method does already set
combine to true so we are able to parse error messages
(#19760) Remove unnecessary stubbing in sun_spec

We do not have to stub the `command` method anymore because it is
nevery called directly. It is only called through the wrapper method
`pkginfo` which we already stub in the specs.
Merge branch 'ticket/stable/19760_fix_sun_provider' into stable

* ticket/stable/19760_fix_sun_provider:
  (#19760) Remove unnecessary stubbing in sun_spec
  (#19760) Use pkginfo method instead of execute
  (#19760) Use combine when running pkginfo