satellite/audit: add tests for concurrent audits

This commit introduces tests that perform multiple concurrent audits
against the same storage node, to make sure that doing so does not
create incorrect outcomes.

Refs: #5495
Change-Id: Iaae49e042306bfa59bdf04c1a1540667488e51e5

Author: thepaul

private/testblobs/bad.go

@@ -15,10 +15,18 @@ import (
 	"storj.io/storj/storagenode"
 )
 
+// ErrorBlobs is the interface of storage.Blobs with the SetError method added.
+// This allows the BadDB{}.Blobs member to be replaced with something that has
+// specific behavior changes.
+type ErrorBlobs interface {
+	storage.Blobs
+	SetError(err error)
+}
+
 // BadDB implements bad storage node DB.
 type BadDB struct {
 	storagenode.DB
-	blobs *BadBlobs
+	Blobs ErrorBlobs
 	log   *zap.Logger
 }
 
@@ -27,19 +35,19 @@ type BadDB struct {
 func NewBadDB(log *zap.Logger, db storagenode.DB) *BadDB {
 	return &BadDB{
 		DB:    db,
-		blobs: newBadBlobs(log, db.Pieces()),
+		Blobs: newBadBlobs(log, db.Pieces()),
 		log:   log,
 	}
 }
 
 // Pieces returns the blob store.
 func (bad *BadDB) Pieces() storage.Blobs {
-	return bad.blobs
+	return bad.Blobs
 }
 
 // SetError sets an error to be returned for all piece operations.
 func (bad *BadDB) SetError(err error) {
-	bad.blobs.SetError(err)
+	bad.Blobs.SetError(err)
 }
 
 // BadBlobs implements a bad blob store.