Edit testing doc and add admonition to Grzegorz blog post on Qt testing

[willingc]

References and relevant issues

Closes #494

Description

This PR edits the testing page. After reviewing each section, I added
information that would be helpful for contributors. An admonition was
added to reference Grzegorz' blog post on avoiding segfaults in Qt tests.

Also rearranged some sections like mocking to group common tasks before specifics.

[TimMonko]

Overall great changes. Was about to play with some testing things (to see if I can replicate the Windows seg faults on my PC), so will use this PR as the resource and report back.

[TimMonko]

Looks great! I like how the new change allows us to evolve by pointing to the necessary page, and not having to maintain the material in two places. Thanks!!

[willingc]

CI error for docs build is unrelated to this PR.

[TimMonko]

not sure how to retrigger circleCI action? I'll just update branch

[TimMonko]

I will say Circle was being finnicky early when I was merging into napari/napari

[willingc]

Thanks @TimMonko.

For the future, you can click through to the Circle CI dashboard and the upper right has a button to rerun Circle CI.

[TimMonko]

Appreciate YOU @willingc

[psobolewskiPhD]

thanks @willingc