Test documentation: replacement of unittest references with pytest framework references

[SRomansky]

Documentation in the freeseer testing guide has become outdated due to the migration from the unittest framework to the pytest framework. This pull request removes references to running unit tests with unittest and instead replaces them with information on how to run the unit tests with the pytest framework.

Closes #467

[dideler]

I think a single reference to unittest should be included. Mention that there are two ways to run the test, using pytest (show how) and using unittest (show how).

[dideler]

Can you give the PR a more descriptive title (what's the documentation update?) and move the issue reference to the PR description? You can refer to the relevant issue at the bottom of the PR description with Closes #467, which will also auto-close that issue when this gets merged.

Then you can squash all your commits into a single descriptive commit (which should follow these commit message guidelines), and then this will probably be ready to merge.

[dideler]

Merged as 599a5196830c1a5f67f894b2161415833f823a97, thanks!