selftests/powerpc: Make context_switch do something with no args

For ease of use make the context_switch test do something useful when
called with no arguments.

Default to a 30 second run, using threads, doing yield, and use any
online cpu. Make it print out what it's doing to avoid confusion.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Anton Blanchard <anton@samba.org>