drivers: base: test: Make property entry API test modular

There is no reason why the KUnit Tests for the property entry API can
only be built-in.  Add support for building these tests as a loadable
module, like is supported by most other tests.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/98388154383df9d4ced73946efd18318aeea50e2.1695820382.git.geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>