Test utils for SAVE and SAVE plugins

[petertrr]

• [ ] Create directory structure from string (tree-like syntax)

    .
    |-- foo
    |-- bar
    |    |-- baz
    |    `-- qux
    `-- stuff
  

• [x] ~For plugins: Should have option to provide some input, apply execCmd to it (simply mocking it's output like cat mockedOutput doesn't work, because e.g. warn-plugin will have important logic of copying input file with comments filtering)~ - MockPlugin is implemented