selftests/powerpc/ptrace: Use more interesting values

The ptrace-gpr test uses fixed values to test that registers can be
read/written via ptrace. In particular it sets all GPRs to 1, which
means the test could miss some types of bugs - eg. if the kernel was
only returning the low word.

So generate some random values at startup and use those instead.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-12-mpe@ellerman.id.au