Improve functionality of managing Testcases and its UI.

[adityachirania]

• As of now the extension only allows to add more testcases, but does not support any feature for deleting a particular test case. It also doesnt allow running a particular testcase only.
• A good user interface can also be provided to a user managing the test cases of a problem through the webview API.
• Any other enhancements in the testing features are welcome.