Load dials env as a pre-launch task for tests

As a developer, I want to run the unit tests via the vscode GUI without having to run the `load_dials_env` prelaunch task, as it is currently configured to do, so that I can develop against beamlines that are unrelated to dials without extra setup and develop in a non-dls_sw environment (e.g. my laptop or a devcontainer).

See below snippet from `launch.json`:

```json
...
"purpose": [
"debug-test"
],
"console": "integratedTerminal",
"preLaunchTask": "load_dials_env",
"env": {
// The default config in setup.cfg's "[tool:pytest]" adds coverage.
// Cannot have coverage and debugging at the same time.
// https://github.com/microsoft/vscode-python/issues/693
"PYTEST_ADDOPTS": "--no-cov"
},
...
```

@DominicOram @dperl-dls is the dials env still a requirement for dodal? If so then maybe we can make `load_dials_env` be a no-op if the environment is unavailable.

## Acceptance Criteria
* The vscode test launch config no longer requires the fake dials env to start