Allow setting thread level for tests

MVAPICH on Ookami doesn't support threading, but we default `MPI.Init` to `threadlevel=:serialized`
https://github.com/JuliaParallel/MPI.jl/blob/a179cf876a52876d297b7e635881dbc7a2f3a81c/src/environment.jl#L81
and this creates a lot of noise when running the tests because each test would print a series of

```
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96
```

The solution should be as easy as setting the default value of `threadlevel` in `tests/common.jl` and always initialise with `MPI.Init(; threadlevel)` in all tests, the only question is _how_ to let the user choose the threadlevel for the tests. Another environment variable?

Alternatively, have a smarter `threadlevel` default value, but can we query whether the MPI library supports threading before `Init`?

Start with src/environment.jl around the default MPI.Init threadlevel and tests/common.jl, then inspect how the tests initialize MPI. Decide how users should select the test threadlevel, such as through a setting or environment variable. Done means all tests use the selected level and avoid repeated warnings on libraries that do not support threading.

Written by the indexing model from the issue text.