Add failed testcases into testcase available for execution

[dvishal485]

Feature request

Failed testcase should be added to a testcase file making it easier to run the code against those failed testcase.

---

Instructions

• Automatically add the failed testcase during submission into failed.txt.
• Overwrite the file everytime the user gets a failed testcase.
• The testcase should not be appended in testcase.txt as it may contain test cases from a different question and also may exceed the test case limit (provided by Leetcode).
• Whenever testcase is added in failed.txt, user should be shown appropriate message with command leetcode-runner-cli -r -t textcase.txt -f [codefile_name] which will execute the failed testcase.
• This should only be triggered on failing submission and not on failing testcase.
• The failed testcase is present as last_testcase (which is a String) in some variant struct of the enum SubmissionResult.
• Make sure the string is non-empty before adding it into failed testcase.

Suggestion

To avoid code repetition, a trait can be used to define shared behaviour of these struct (the the variants of enum store) so they may invoke save_failed_testcase() method which does the job.