contributing.md instructions for creating tests

[tukib]

friendlier instructions for creating tests, with links to Jest documentation.

[Fayti1703]

This doesn't catch the throw unless you wrap the code to test in a function:

Suggested change

	expect(log2(0)).toThrow();
	expect(log2(-1)).toThrow();
	expect(() => log2(0)).toThrow();
	expect(() => log2(-1)).toThrow();

(I know this is just example code, but still.)

[seanmakesgames]

@Fayti1703 @oolu4236 @ellipsisasterisk and I checked this out on codespaces and also updated the branch to remove conflicts.

The changes to the jest config file did not break pnpm test