selftests/powerpc: Fix TM tests when CPU 0 is offline

Several of the TM tests fail spuriously if CPU 0 is offline, because
they blindly try to affinitise to CPU 0.

Fix them by picking any online CPU and using that instead.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200813013445.686464-1-mpe@ellerman.id.au