Tests code improvements

[rastorc3v]

• var replaced with const

• anonymous functions replaced with arrow functions

• PI value replaced with Math.PI constant

• equal replaced with strictEqual since the first one was deprecated

• [x] tests

• [x] linter

[msimerson]

equal replaced with strictEqual since the first one was deprecated

The better way to do this is by loading assert like this: const assert = require('assert/strict'), or if we're willing to drop support for older versions of node: const assert = require('node:assert/strict').