[test] Avoid reusing a.mjs for all js tests. (#7534)

Instead create `<filename>.mjs` named after each test.

Test output now looks like this:

```
$ ./check.py --binaryen-bin=emcc-build/bin binaryenjs_wasm
warning: Binaryen not found (or has not been successfully built to bin/ ?
executing:  /usr/bin/node -e process.stdout.write(typeof WebAssembly)

[ checking binaryen.js testcases (/usr/local/google/home/sbc/dev/wasm/binaryen/emcc-build/bin/binaryen_wasm.js)... ]

executing:  /usr/bin/node atomics.mjs
executing:  /usr/bin/node closed-world.mjs
executing:  /usr/bin/node copy-expression.mjs
executing:  /usr/bin/node custom-section.mjs
...
```

Previously:

```
$ ./check.py --binaryen-bin=emcc-build/bin binaryenjs_wasm
warning: Binaryen not found (or has not been successfully built to bin/ ?
executing:  /usr/bin/node -e process.stdout.write(typeof WebAssembly)

[ checking binaryen.js testcases (/usr/local/google/home/sbc/dev/wasm/binaryen/emcc-build/bin/binaryen_wasm.js)... ]

atomics.js
executing:  /usr/bin/node a.mjs
closed-world.js
executing:  /usr/bin/node a.mjs
copy-expression.js
executing:  /usr/bin/node a.mjs
custom-section.js
executing:  /usr/bin/node a.mjs
debug-info.js
executing:  /usr/bin/node a.mjs
debug-names.js
executing:  /usr/bin/node a.mjs
emit_asmjs.js
...
```