OpTestPCI: Correct subprocess usage so we emit diff in test case error
OpTestPCI: Correct subprocess usage so we emit diff in test case error

While the output of the diff was appearing in the output of the test
run, it was not being passed to the unittest framework as an error
message, which means that it wasn't shown in test execution summary.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework