MPIPreferences + Test

Probably a more general/upstream issue but I'll post it here nonetheless, because this is where I encountered it. (UPDATE: https://github.com/JuliaLang/Pkg.jl/issues/3389)

**In short, the issue is that `] test` doesn't seem to respect the `JULIA_LOAD_PATH` variable and thus global preference files.**

On our cluster, we provide a global `Project.toml` (say at `/some/global/path/Project.toml`) in which we set the MPI preferences, specifically
```
[extras]
MPIPreferences = "3da0fdf6-3ccc-4f1b-acd9-58baa6c99267"

[preferences.MPIPreferences]
_format = "1.0"
abi = "OpenMPI"
binary = "system"
libmpi = "libmpi"
mpiexec = "mpiexec"
```

We then automatically set `JULIA_LOAD_PATH=:/some/gobal/path` when the user loads the julia module. This works just fine in a regular Julia session where one gets
```
LOAD_PATH == ["@", "@v#.#", "@stdlib", "/some/global/path"]
```

However, when running `] test` the `JULIA_LOAD_PATH` env variable is ignored and one only gets
```
LOAD_PATH == ["@", "/tmp/jl_KwLRNl"]
```

This means that the global MPI preferences are not respected, leading to undesired behavior and, in my specific case, an indefinite hang.