tests: drivers: stepper: limit test platforms

The stepper driver test cases only specify integration
platforms, which whitelists every other board in tree
for full CI runs. This is not intended as some platforms
are too small to run the test cases.

Whitelist target platforms using platform_allow instead.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>