Here's the current list of valid emoji, including upcoming ones being added in the next revision.[2]
A reasonable test for passwords is to run them through an IDNA checker, which checks whether a string is acceptable as a domain name component. This catches most weird stuff, such as mixed left-to-right and right-to-left symbols, zero-width markers, homoglyphs, and emoji.
[1] https://www.washingtonpost.com/news/the-intersect/wp/2015/02... [2] http://unicode.org/emoji/charts-beta/full-emoji-list.html