Move wasm_for_tests/ into the wasm workspace

[james-chf]

Closes #102

This PR attempts to move our test wasms (wasm_for_tests/wasm_source) into the wasm workspace (wasm/test_wasms). Previously test wasms were prebuilt directly under wasm_for_tests/*.wasm - since wasm/*.wasm globs our production wasms, the test wasms are now built under wasm/test_wasms/built/*.wasm. This is just to get things working, not actually sure where is best to put them.

Low priority, may be better to tidy up references to the wasm_for_tests directory first in our code, as it's referenced in many places.

[james-chf]

Moving this works straightforwardly enough but the current wasm_for_tests path is hardcoded/references in disparate places, we should probably clean that up a bit first before doing this move (e.g. as in #893)