Improve contributor docs on testing

[snOm3ad]

Describe the Bug

When testing wasm-bindgen using cargo test --target wasm32-unknown-unknown the tests located in tests/wasm/ fail with a somewhat cryptic error message:

Caused by:
    0: failed to generate bindings for import of `__wbindgen_placeholder__::__wbindgen_link_91767d44553c09e6`
    1: wasm-bindgen needs to be invoked with `--split-linked-modules`, because "./not-found.js" cannot be embedded.
       See https://rustwasm.github.io/wasm-bindgen/reference/cli.html#--split-linked-modules for details.
error: test failed, to rerun pass `--test wasm`

It is not immediately obvious that the fix is to set environment variable WASM_BINDGEN_SPLIT_LINKED_MODULES before running the test suite.

[flumm]

should be improved (if not fixed) by #3629

[daxpedda]

Thanks @snOm3ad for reporting it and @flumm for fixing it!

Fixed by #3629.