Refactor(fixtures): Improve test fixture code

Not for this PR, but the fixtures stuff should be reviewed, as this could probably just be a test helper that handles the skip itself, vs. having the:

```go
if testutil.SkipIfNoAPIKey(providerName) {
    t.Skip("FOO_API_KEY not set")
}
```

everywhere

_Originally posted by @peteski22 in https://github.com/mozilla-ai/any-llm-go/pull/26#discussion_r2765804492_
            