I actually don't know if there are fonts to cover every single unicode character (as in there may be characters that literally don't have a font that supports it), but I could be wrong.
I'll suggest a ridiculous hack, but it could end up being a cool little project.
You can download all of the unicode characters from the unicode website, and then write a script to iterate through them all.
This site is a pretty good resource for finding fonts that support a character: https://www.fileformat.info/info/unicode/char/0041/fontsuppo...
Just switch out the second to last param (0041) in that URL with the character code, make a GET request to the page, and then scrape the html for a font. Then do some kind of a distinct() on those fonts and you might have a pretty good list (you might DOS the site though).