Feature Request: Support `-p` flag (test parallelism) like `go test`

Hi! 👋

`go test` supports the `-p` flag to control the **number of packages tested in parallel**:

```
go test -p 1
```

I would like `gotestsum` to support the same flag:

```
gotestsum -p 1
```

This is especially useful when running tests that touch shared resources (DB, Redis, filesystem, etc.), in CI environments where limiting parallelism is important, or when test packages must run one-by-one (in my case).

Thanks for the great tool — this feature would help a lot with deterministic test execution. 🙏