Add tests for fast_atof

The custom implementation for fast_atof currently lacks test coverage,
making it susceptiple to regressions with future changes. Add some
basic test coverage, in addition to a few documented cases that are
known to fail.
Merge pull request #374 from jdduke/test_atof

Add tests for fast_atof
Merge remote-tracking branch 'upstream/master' into cmake-make-test-support
code/fast_atof.h