Previously, when calling `BigFraction::from_string()`, the fractional
part of the number was always treated as positive. This led to an
incorrect result if the input string was negative.
Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.