New answers tagged resources
I'd recommend searching for some open source math books; there are quite some on Github, and probably more elsewhere. That way, you can somehow choose the kind of formulae you want to test; eg., topology (with much algebra), mathematical modelling (linear algebra, differential equations), combinatorics, or much more abstract stuff like the HoTT book with a ...
the file testmath.tex is part of the documentation for the amsmath package. it's in tex live, on ctan, or available via a link on the page http://www.ams.org/tex/amslatex (under "additional documentation}. the content is kind of "random", but has a good variety of examples and has been used, among other things, for stress testing of new math fonts.
This could be a useful collection of latex examples: Latex-examples
Top 50 recent answers are included