Factor in engine flags to the name of an engine for uniqueness

I was curious to test some flags against each other with Wasmtime to determine their effect on performance today, so I passed the same `*.so` as the `-e` flag to two collection runs with different `--engine-flags` values. When running `effect-size`, however, it failed with:

```
Error: Can only test significance between exactly two different engines. Found 1 different engines.
```

I then renamed the same `*.so` to two different files and the comparison came out ok, so this might be a case where factoring in the flags to the uniqueness calculating and/or printing later on would help.