tests: ztest/base: fix incorrect userspace filtering

The filter here should be used to filter for capability:
whether the platform configuration supports userspace. And if
it does support userspace, we then enable CONFIG_TEST_USERSPACE
(and thus CONFIG_USERSPACE) for testing. We should not be
filtering for whether userspace is enabled, but should really
be filtering for whether userspace is supported.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>