fixes#21441
When compiling for JS, nimscript config files have both `defined(js)`
and `defined(nimscript)` be true at the same time. This is required so
that the nimscript config file knows the current compilation is for the
JS backend. However the system module doesn't account for this in some
cases, defining JS-specific code or not defining nimscript-specific code
when compiling such nimscript files. To fix this, have the `nimscript`
define take priority over the `js` one.
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)
6cc50ec316