add testcases for nested list coercion

[jjergus]

Add some test cases to cover coercion of list values, especially nested lists where some non-list values are involved. These exercise the following special case from the spec:

If the value passed as an input to a list type is not a list and not the null value, then the result of input coercion is a list of size one, where the single item value is the result of input coercion for the list’s item type on the provided value (note this may apply recursively for nested lists).

Most of these examples are copied directly from the spec. However, one of them actually coerces to a value different from what the example in the spec says, which I believe to be a mistake in the spec: graphql/graphql-spec#515

@IvanGoncharov what do you think?

[IvanGoncharov]

@jjergus Thanks for PR 👍
I think it better to have describe for for GraphQLList and just implement all test from this table: http://facebook.github.io/graphql/June2018/#sec-Type-System.List

[IvanGoncharov]

I think it's better to use existing naming convention, e.g.

graphql-js/src/utilities/__tests__/coerceValue-test.js

Lines 131 to 134 in 7bc632a

	it('returns value for integer', () => {
	const result = coerceValue(1, GraphQLFloat);
	expectValue(result).to.equal(1);
	});

graphql-js/src/utilities/__tests__/coerceValue-test.js

Lines 75 to 78 in 7bc632a

	it('returns null for null value', () => {
	const result = coerceValue(null, GraphQLInt);
	expectValue(result).to.equal(null);
	});

So instead of correctly coerces a nested list value it could be return value for nested list.
Same goes for other testcases

[jjergus]

Thanks for the review! I added more test cases and made the descriptions consistent with the rest of the file. I'm still keeping the nested list test cases separate from the non-nested ones, since it seems to be easier to read that way.

[mjmahone]

This seems like a good set of tests. Thanks!