Merge gen_wasm tests with existing gen tests

[shua]

from #7514 (comment)

It looks like we are duplicating some tests in crates/compiler/test_gen for the gen_wasm case and then gen_llvm+gen_dev case. From the git log, these were separated due to pointer width issues, which may no longer be an issue.

Try merging the duplicated tests, eg move wasm_str.rs tests to gen_str.rs .