[test] introduce script scopes around expensive memory tests

[rossberg]

Fix #1698. This tweaks the generators for bulk memory/table tests to insert (script ...) scopes around tests and extends the JS converter to turn these into block scopes.

This is likely to break existing .wast consumers that do not recognise script scopes. The simplest fix is to ignore them by treating them as equivalent to their contents.

[titzer]

Does it help to split up these tests into multiple .wasts? In general, splitting up the many large spec tests helps ergonomics of the test suite.

[rossberg]

Personally, I'm not a big fan of having a gazillion micro test files. But in any case, these are generated, so that would require a more significant rewrite of the JS generator scripts, that somebody would have to volunteer doing.