Updating UpdateDependency test for all package managers + tests fixes

[eranturgeman]

• [x] All tests passed. If this feature is not already covered by the tests, I added new tests.
• [x] This pull request is on the dev branch.

• [x] I used gofmt for formatting the code before submitting the pull request.

  This PR introduces improvements for UpdateDependency test: 1) added missing testcase for pipenv 2) added a check to verify dependency update in test cases where fix is suppose to happen 3) added informative comments for test cases 4) added a defer call for deleting tmp directories created during tests

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[eranturgeman]

Looks great! Just note that the previous tests also covered the whole "find descriptor file" process per technology, and if you'd like to do more of an integration test, this functionality should be checked as well.

Thanks @EyalDelarea ! I unified all the "find descriptor files" functions for the different PMs into a single function that is being checked separately. This change and new test were introduced in this PR: https://github.com/jfrog/frogbot/pull/656 Therefore I could take it our from the test that checks every PM