fixes#26088
in https://github.com/nim-lang/Nim/pull/25772, `beginStores` requires
`newLen` to be passed for setting up the new length. So `streams.nim`
must make the string length exactly newLen.
Prefer moving tests to a non-js directory so that they get tested across all backends automatically.
Ideally, tests/js should be reserved to code that only makes sense in js.
Note also that tests for a js specific module (e.g.: std/jsbigints) belong to tests/stdlib, (e.g.: tests/stdlib/tjsbigints.nim)