Add Wasm tests running with wasm-bindgen-test

[jakubadamw]

running 7 tests
test wasm32_datetime::nil_is_at_unix_epoch ... ok
test wasm32_datetime::default_is_nil ... ok
test wasm32_datetime::test_timestamp ... ok
test wasm32_datetime::test_datetime ... ok
test wasm32_datetime::test_order ... ok
test wasm32_datetime::test_source ... ok
test wasm32_datetime::test_dynamic ... ok
test result: ok. 7 passed; 0 failed; 0 ignored

Unfortunately, I had to add them as a new test module because the runner won't pick them up otherwise. See the last sentence of this guide.