Revert previous changes because of wrong gas calculation in vm tests

In https://github.com/ethereum/cpp-ethereum/commit/efaa2977595f49d1928f048194c23457902cd21d gas was calculated after vm::go was called, leaving gas to 0 if an exception was thrown. This is corrected.
Additionally I changed the path for the vm tests if the environmental variable ETHEREUM_TEST_PATH is set.