test: fix file prefix discovery

test: fix file prefix discovery

Config file has moved, but the tests weren't updated to point to
its new location. Update the code to find current prefix.

Also, this function is duplicated across multiple tests, so move it
into process.h and force compile failures for any attempts to use
it on platforms other than Linux.

Fixes: adf1d867361c ("eal: move runtime config file to new location")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Reshma Pattan <reshma.pattan@intel.com>