bug: asdf plugin test logic does not match documentation re: version

### Describe the Bug

The Go code for `pluginTestCommand`:

```go
// grab first version returned by list-all callback if no version provided as
// a CLI argument
if toolVersion == "" {
toolVersion = allVersions[0]
}
```

But the docs say:

> If optional [--asdf-tool-version ] is specified, the tool will be installed with that specific version. Defaults to asdf latest

### Steps to Reproduce

running the following fails:
```bash
asdf plugin test golang https://github.com/asdf-community/asdf-golang.git go version
```

while the following passes:
```bash
asdf plugin test golang --asdf-tool-version $(asdf latest golang) https://github.com/asdf-community/asdf-golang.git go version
```

### Expected Behaviour

running `asdf plugin test` without a setting `asdf-tool-version` should use the latest stable version.

### Actual Behaviour

It uses the first version, which may no longer match the tests.

### Environment

```shell
irrelevant, the code pasted above clearly shows the disparity.
```

### asdf plugins affected (if relevant)

_No response_

Search the Go code for pluginTestCommand and inspect how it selects a version when --asdf-tool-version is absent. Reproduce the issue with the two asdf plugin test commands shown, then compare the selected version with asdf latest. Done means the no-argument command uses the latest stable version while an explicitly supplied version still works.

Written by the indexing model from the issue text.