test: proxy engine edge cases + storage/server minio retry

Cover denied, HEAD (cached/uncached/denied), stale-on-error, circuit-open,
and mutable revalidation (304) paths. proxy ~8->58%.

internal/storage/storage_test.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"strings"
 	"testing"
+	"time"
 
 	"git.unkin.net/unkin/artifactapi/internal/testsupport"
 )
@@ -19,7 +20,13 @@ func TestMain(m *testing.M) {
 	if err != nil {
 		os.Exit(m.Run())
 	}
-	s3, err := NewS3(conn.Endpoint, conn.AccessKey, conn.SecretKey, "test-bucket", false, "")
+	var s3 *S3
+	for i := 0; i < 20; i++ { // MinIO can report ready before bucket ops succeed
+		if s3, err = NewS3(conn.Endpoint, conn.AccessKey, conn.SecretKey, "test-bucket", false, ""); err == nil {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
 	if err != nil {
 		terminate()
 		panic(err)