Since the command object is created using the TestSuite as argument, we
can keep a reference to the test suite's scoped T() instance for the
command's lifecycle, and reuse it later during `Assert`, instead of
needing to pass it as argument when invoking the function.
When the PACKER_PLUGIN_PATH envvar is defined in the environment, Packer
uses it as the source of truth for the directories in which to look for
plugins to load.
Previously, we used to support multiple directories separated by the
OS-specific path separator (i.e. : on UNIX, ; on Windows).
Since this changed, and Packer returns an error to the user, we make
sure that this is well-documented and tested through this extra test.
Lucas Bajolet
Wilken Rivera