Removing needless includes from expression tests and adding more tests.

Summary:
The previous quick push was to properly show things like scientific notation.
Mul.tex() was omitting the multiplication sign before any Pow, even if that
Pow's base was numerical. So parse("4*10^5").tex() would equal "410^{5}".

Test Plan: src/expressiontools/test.html

Reviewers: alpert

Differential Revision: http://phabricator.khanacademy.org/D2603