software node: introduce CONFIG_KUNIT_DRIVER_PE_TEST

Currently the property entry kunit tests are built if CONFIG_KUNIT=y.
This will cause warnings when merged with the kunit tree that now
supports tristate CONFIG_KUNIT.  While the tests appear to compile
as a module, we get a warning about missing module license.

It's better to have a per-test suite CONFIG variable so that
we can do selective building of kunit-based suites, and can
also avoid merge issues like this.

Fixes: c032ace7

 ("software node: add basic tests for property entries")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>