Feature Request: test- and benchmark-arguments per target configuration

As per stack 2.3.1, it seems to only be possible to define **project-wide** `test-arguments` and `benchmark-arguments` as per:

```yaml
build:
test-arguments: "--fail-fast"
benchmark-opts:
benchmark-arguments: "--output bench.html"
```

However these apply to _all_ test or benchmark targets defined in the `package.yaml`.

I think there's a valid use-case where multiple test or benchmark targets may be specified in the configuration but require different arguments.

For example, in the event of benchmarks using Criterion, it may be useful to have different run times or HTML output files for each configuration, perhaps something like this:

```yaml
benchmarks:
fast-benchmarks:
arguments: "--output bench-fast.html"
slow-benchmarks:
arguments: "--time-limit 60 --output bench-slow.html"
```

With the current project-level benchmark arguments, the HTML output files will all be called the same thing and collide with each other, and there's no finer-grained control of benchmark time limits.