selftests/powerpc: Give the bad_accesses test longer to run

On older systems this test takes longer to run (duh), give it five
minutes which is long enough on a G5 970FX @ 1.6GHz.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-2-mpe@ellerman.id.au