[dvsim] Simulation reports moved when specifying proj_root

Greetings @rswarbrick ,

By default, the simulation reports(html, json) are under the folder `scratch//_sim_/reports/`.

However, if the switch `--proj-root` is set, the `scratch` folder will be under `proj_root`, but the `reports` folder disappears, and the reports will be directly under where the `sim_cfg.hjson` file locates.

Taking uart as an example:
```
// $(pwd) = opentitan/
python3 util/dvsim/dvsim.py hw/ip/uart/dv/uart_sim_cfg.hjson -i uart_smoke -pr $(pwd)
```
Now the simulation reports are no longer under `scratch` and is under `opentitan/hw/ip/uart/dv/` and the `reports` folder will not be created.

Reproduce the issue with util/dvsim/dvsim.py and hw/ip/uart/dv/uart_sim_cfg.hjson using the documented --proj-root command. Trace how the project root and report directory are resolved; done means HTML and JSON reports remain under the proj_root scratch//_sim_/reports/ path.

Written by the indexing model from the issue text.