fix the regressions caused by PR#20

The most important issue was that PR#20 removed the possibility to use
a test driver script. In eWoms, this script is responsible for
(fuzzily) comparing the results of the test runs with reference
solutions, to test the simulators in parallel and compare the results,
and to make sure that the --help command continues to work as
intended.

the approach taken to bring this functionality back is to specify the
test driver script and its parameters using the new macro
'opm_set_test_driver(${DRIVER_COMMAND} ${DRIVER_DEFAULT_ARGUMENTS})'
before the first call to 'opm_add_test'. If no test driver is
specified, the binary is run 'naked', so nothing should change in this
case.

this patch requires the changes for opm-cmake to be merged first.

tests/run-vtu-test.sh

@@ -12,7 +12,7 @@ usage() {
     echo "Usage:"
     echo
     echo "runTest.sh TEST_TYPE TEST_BINARY [TEST_ARGS]"
-    echo "where TEST_TYPE can either be --plain,  --simulation or --parallel-simulation=\$NUM_CORES (is '$TEST_TYPE')."
+    echo "where TEST_TYPE can either be --plain, --simulation or --parallel-simulation=\$NUM_CORES (is '$TEST_TYPE')."
 };
 
 validateResults() {