Use wasm-compatible asserts in tests

[cwfitzgerald]

Because we want to have expected-to-fail tests (due to driver bugs or internal bugs), we want all tests to be recoverable, even on wasm. Currently we achieve this by catching panics from various assertions. However, because wasm doesn't support stack unwinding, we can't do this.

We should switch to assert calls that bail out of the test either through early returns, or no returns at all. This way we can properly recover after errors on wasm as well.