Passing host args to SpiderMonkey causes module tests to fail

We end up running the shell with
```
    --module
    [hostarg]
    /tmp/HqmLu073pu5L64GWXI66/await-identifier-module-1.js
```
instead of

```
    [hostarg]
    --module
    /tmp/HqmLu073pu5L64GWXI66/await-identifier-module-1.js
```
causing
```
Error: can't open [hostarg]: No such file or directory
```
Fix to eshost incoming.