test: drop golang.org/x/sync dependency (errgroup → sync.WaitGroup)

[VojtechVitek]

Removes the golang.org/x/sync dependency, which was only used by the test suite (errgroup in local_counter_test.go). The concurrent test goroutines now report failures directly via t.Errorf (safe for concurrent use), so errgroup's error aggregation isn't needed.

This is one part of reducing the module's dependency footprint as suggested by @lrstanley in #57. Credit for the idea goes to them.

Note: x/sync is a test-only dependency, so it doesn't propagate to consumers' builds — the benefit here is a tidier go.mod rather than a change to the dependency graph downstream users see. The zeebo/xxh3 part of #57 is handled/discussed separately.

• go test ./... passes
• go mod tidy drops golang.org/x/sync from go.mod/go.sum

🤖 Prepared with the help of Claude (AI), reviewed before opening.

[github-actions]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
               │ master.txt  │               pr.txt               │
               │   sec/op    │   sec/op     vs base               │
LocalCounter-4   38.80m ± 1%   39.47m ± 2%  +1.71% (p=0.015 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.846Mi ± 0%   2.853Mi ± 0%  ~ (p=0.436 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.5k ± 0%   121.5k ± 0%  ~ (p=0.447 n=10)