merge wasm_str tests into gen_str

This change merges the wasm_str.rs and gen_str.rs tests.
Any tests that could be run on all three backends now are, in addition some
tests were added that just fail whenever some unimplemented things get implemented
so the implementer will be forced to start testing the new code.

crates/compiler/test_gen/src/tests.rs

@@ -22,7 +22,6 @@ pub mod gen_str;
 pub mod gen_tags;
 pub mod gen_tuples;
 mod helpers;
-pub mod wasm_str;
 
 #[cfg(feature = "gen-wasm")]
 pub mod wasm_linking;