Modify tests to fix flakiness

[blazyy]

Multiple tests inside test_mdutils.py were found to be flaky, i.e. tests that both pass and fail despite no changes to the code or the tests itself.

Using the pytest-flakefinder plugin, when the tests were re-run more than once, multiple tests inside test_mdutils.py were failing. Upon looking at the code, the reason was because the working directory was being changed due to the various calls to os.chdir(tmp) inside test_fileutils.py. This meant the tests could not locate Test_file.md in the directory they were meant to be in.

To fix this, I just removed the calls to os.chdir(tmp) in test_fileutils.py and modified MarkDownFile inside fileutils.py to include another parameter called tmp, which indicates whether or not the markdown file should be created in the temporary directory or not. If this is not empty, the code is modified in such a way that the temporary files are created in the temporary directory without calls to os.chdir(tmp). This is done by including the entire path that the file is supposed to be in.

To reproduce:

1. Install pytest-flakefinder by following the instructions here.
2. At root directory, run pytest --flake-finder tests

After implementing the fix, all tests pass successfully, both with and without the pytest-flakefinder plugin being used.

In general, flaky tests are a pain to fix with regards to locating the root of the actual problem, so it's a good idea to fix them when you detect them. I'm aware that the tests might not be re-run at all, but this change makes the entire module more robust and future-proof so it's just a small fix for you to consider.

[didix21]

@blazyy Thanks for your contribution!