This commit switches the 'test_ecl_output' unit test to using the
EclIO::ESmry class from OPM-Common. We add simple alternative
implementations of ecl_get_field_var() and ecl_get_general_var() in
terms of ESmry to avoid rewriting the test code itself.
As class ESmry does currently not support reading non-unified
(separate) summary files (.S000n), we temporarily switch the deck to
using unified output.