Some tests take longer than others, and so may want to set a custom
timeout so that they pass, without increasing the timeout for all other
tests. For example, this is done in WPT.
Add an `internals.setTestTimeout(milliseconds)` method that overrides
the test runner's default timeout for the currently-run test.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
