Added running tests file to shared docs.

Author: BethanyG

exercises/shared/tests.md

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+# Tests
+
+You can run the incluyded tests by typing `pytest <exercisename>_test.py` on the command line from within the exercise's directory.
+
+You can also tell Python to run the pytest module on the command line from either within the exercise directory or with a path to the exercise directory.
+`python -m pytest <exercisename>_test.py` from within the exercise directory.
+
+`python -m pytest /fully/qualified/path/to/<exercisename>/` OR `python -m pytest realtive/path/to/<exercisename>` from a non-exercise directory.
+
+Many IDE's and code editors also have built-in spport for using PyTest to run tests.
+
+[Visual Studio Code](https://code.visualstudio.com/docs/python/testing)
+[PyCharm Professional & Community Editions](https://www.jetbrains.com/help/pycharm/pytest.html#create-pytest-test)
+[Atom](https://atom.io/packages/atom-python-test)
+[Spyder](https://www.spyder-ide.org/blog/introducing-unittest-plugin/)
+[Sublime](https://github.com/kaste/PyTest)
+[vim-test](https://github.com/vim-test/vim-test)
+
+See the [Python tests page](https://exercism.io/tracks/python/tests) for more information.
+
+### Common `pytest` options
+
+- `-v` : enable verbose output.
+- `-x` : stop running tests on first failure.
+- `--ff` : run failures from previous test before running other test cases.
+
+For other options, see `python -m pytest -h`. PyTest documentation can be found [here](https://docs.pytest.org/en/6.2.x/getting-started.html).