[DON'T SQUASH] Update params trailing comma tests

[leobalter]

Closes #822

[leobalter]

cc @littledan

[leobalter]

these tests are invalid. I'm fixing this PR soon

function* f1() {
    length = arguments.length;
};
f1(1,).next();

this has no connection with the trailing commas on the parameters list, but the arguments object and how the length is formed to correspond the given arguments list.

Instead of adding this extra case, I'm removing all the params-trailing-comma-arguments.js files.

[leobalter]

In the meanwhile, I need to search for tests asserting the well formed arguments.length

[leobalter]

test/language/expressions/call/trailing-comma.js is the only other test for trailing comma in a call expression, but there's no tests for the arguments.length

[leobalter]

I can add more cases but that would create more and more files and it's becoming way larger on every new addition.

I suggest we follow up with new PRs to have easier reviews.

[gnarf]

This is some pretty dense stuff, but I read over all the .template and .case files, nothing stands out to me, the info blocks seem to match up, etc.

I'd still prefer if @littledan, @rwaldron, or someone else would take a good look here (even if after we merge it)

[leobalter]

these tests will open the way for me to add more stuff on async generators, early errors, etc. I'll have this merged but already assuming the responsibility to fix anything else we find wrong.