[lit] Fix test that relied on "single process" mode

The shtest-inject test relied on being executed in "single process" mode
and started to fail with a `PicklingError` after it was removed:
```
  Can't pickle <class 'lit.TestingConfig.CustomFormat'>: attribute
  lookup lit.TestingConfig.CustomFormat failed
```

This happened because the test config has to be serialized to the worker
process, but apparently the `CustomFormat` class defined inline is not
serializable.

This change allows passing the tested functionality (preamble_commands)
directly to `lit.formats.ShTest` so we can use it directly in the test.