Use `inDelta` not `equals` for double comparisons in polygon area test

Fixes this:
```
    area Polygon
      ✗ lune
        » expected 3.141592653589793,
  		got	 3.1415926535897927 (==) // area-test.js:50
  ✗ Broken » 2546 honored ∙ 1 broken (9.349s)
  npm ERR! Test failed.  See above for more details.
npm ERR! not ok code 0
```