Ensure that test titles are strings before sanitizing them for screenshot names

[jennifer-shehane]

• Close 'Cannot read property \'replace\' of null' from sanitize-filename err being thrown #4310
• Write tests for many non-string values to ensure screenshot is taken.

[kuceb]

@jennifer-shehane why are we allowing non-string test titles anyways? If we error earlier then we wouldn't need to add this logic or ever account for non-string test names again.

[jennifer-shehane]

@bkucera They already work. They work fine when running tests in Cypress and other frameworks, it's just when we try to translate the title to a screenshot filename that it borks.

[kuceb]

@jennifer-shehane how does it look in the dashboard though? reporters? I don't think we handle non-strings anywhere else really. If there's a discussion about this (allowing non-string test titles) somewhere i'd like to read it

[jennifer-shehane]

We're converting them all to strings, so it wouldn't display any other way in the Dashboard. I have no idea how reporters handle it.

[brian-mann]

@bkucera regarding test titles which are purely null or undefined that's a separate discussion.

undefined will not work in JSON, whereas null will. more than likely mocha shits the bed if it's a mess, so we may not handle it because it may not be possible.