Increase test coverage.

[korsbo]

We really should be testing each and every feature of Latexify and currently we are not.

Test generation is made easy by a macro that we supply:

using Latexify
@Latexify.generate_test latexify("x/y")

generates a test and puts it in your clipboard to be pasted:

@test latexify("x/y") == 
raw"$\frac{x}{y}$"

One just have to make sure that the test does actually do what was expected. For markdown, one must also replace the raw with Markdown.MD and get the first newline right.

[korsbo]

This has become increasingly relevant after I stopped testing using ParameterizedFunctions and DiffEqBiological in #117.