1677703 - Fix incorrect RetVal types specifying `0` as type in memory spec

Status: NEW

(DevTools :: Framework, task, P3)

Description

[Julian Descottes [:jdescottes]]

Spotted while working on Bug 1673897.

Most specs only ever use a string type when creating RetVal objects. But in the memory spec, we have a few methods with invalid RetVal calls:
https://searchfox.org/mozilla-central/search?q=RetVal%280&path=devtools&case=false&regexp=false

They all pass 0 as first argument, and then a string (which looks like the actual type).
But RetVal takes a single type argument.

We should fix them and remove the string type check added in isArrayType in D96941 (potentially perform it in the constructor instead)