Adds more edge tests cases for APR

In response to https://github.com/lmp/croovy_finance/commit/a25de9d52f6a25c5d48d4cf7c2d677c8d3d9d0c5#commitcomment-3843409
Replaces decimals with exponential notation

Easier to read, as suggested in
https://github.com/lmp/croovy_finance/commit/28df01a56bdf9da3c91cc02dd921245687565e91#commitcomment-3843327