[test] Replace the non-existent "test" module

[redianthus]

Fix #1502.

Where possible I used "spectest". In some cases I needed a mutable i32 global which doesn't exists in "spectest" so I added a module and registered it.

cc @rossberg

[sunfishcode]

An implementation should be able to perform validation without linking, and without any knowledge of the specific things instances will eventually be linked with. That seems valuable to continue to test.

[rossberg]

@sunfishcode, hah, good point!

[redianthus]

@sunfishcode yes sure, I can add dedicated tests for that, with a comment explaining their goal ?

[sunfishcode]

Comments would be great, however in any case I'd still recommend not switching these tests to "spectest", because implementations shouldn't actually be consulting the actual "spectest" here, and it would seem to risk confusion about whether they should or could.