confusing xfail label

[stellaprins]

Since introducing the emojis XFAIL looks a lot like ❌ FAIL. Furthermore, people not familiar with pytest may be confused by the use of 'XFAIL'. @robertvi suggested using N/A instead.

• Keep original labels but add "expected fail" to XFAIL cases in collapsible content as suggested by @pgleeson.

[stellaprins]

Suggested changes

[stellaprins]

Done as part of PR #70