test: move some functions into another file (pingcap#50984)

Author: lance6716

br/pkg/lightning/backend/external/BUILD.bazel

@@ -62,6 +62,7 @@ go_test(
         "engine_test.go",
         "file_test.go",
         "iter_test.go",
+        "misc_bench_test.go",
         "onefile_writer_test.go",
         "reader_test.go",
         "sort_test.go",