allow multiple paths in test_path attribute of browserstack.json

In browserstack.json, we can now pass, multiple test paths
    "test_path": ["js/tests/index.html","js/tests/index2.html"],
Commit ensures we are backward compatible and accept one test_path:
    "test_path": "js/tests/index.html"

- Current feedback on cli, doesn't mention which test_path is running in which browser, it just mentions the browser, I can add it in if needed.