Run test dir as folder type in ablunit.json config

[kenherring]

Currently when running tests for a directory the test config json contains something like this with an entry for each file.

  "tests": [
    {
      "test": "src/dir/file1.p"
    },
    {
      "test": "src/dir/file2.p"
    },
    //... and so on
    {
      "test": "src/dir/file99.p"
    }
  ]

This PR aims to use the folder key instead, like this.

  "tests": [
    {
      "folder": "src/dir"
    }
  ]