selftests/powerpc: Hoist helper code out of eeh-basic

Hoist some of the useful test environment checking and prep code into
eeh-functions.sh so they can be reused in other tests.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201103044503.917128-1-oohall@gmail.com