bisync: fix flaky TestBisyncConcurrent by increasing random name entropy

The temp directory name used random.String(2) giving only 676 possible
values. When multiple concurrent tests started in the same second, they
shared the same timestamp prefix, causing name collisions and shared
temp directories. This led to lock file conflicts, listing file races,
and file deletion errors.

Increase to random.String(8) to make collisions effectively impossible.

cmd/bisync/bisync_test.go

@@ -329,7 +329,7 @@ func testBisync(ctx context.Context, t *testing.T, path1, path2 string) {
 
 	baseDir, err := os.Getwd()
 	require.NoError(t, err, "get current directory")
-	randName := time.Now().Format("150405") + random.String(2) // some bucket backends don't like dots, keep this short to avoid linux errors
+	randName := time.Now().Format("150405") + random.String(8) // some bucket backends don't like dots, keep this short to avoid linux errors
 	tempDir := filepath.Join(os.TempDir(), randName)
 	workDir := filepath.Join(tempDir, "workdir")