--evm-dump-dir to have default value

[winsvega]

Currently has to type the path for --evm-dump-dir arg.
can have the default value that will output the vm trace next to generated test .json fixture or to a debug folder in the repo itself.

[raxhvl]

Often I dump the log in repo root and have to remember to delete it manually to prevent git from tracking it. How about a git ignored .debug folder?