Replace NvimWebDeviconsHiTest command with checkhealth

[gegoune]

@alex-courtis what do you think about replacing NvimWebDeviconsHiTest to :checkhealth nvim-web-devicons? Can keep the same output but will get whatever benefits :che gives us for free.

[alex-courtis]

Sounds good. Need to brainstorm what else could go in there...

[gegoune]

See #554