feat(tidb): test pipelineddmltest package (#2859)This recently added package `tests/realtikvtest/pipelineddmltest` is nottested.https://github.com/pingcap/tidb/blob/9b6db9d47a0b9dc8fe3c726626ce0428a3d3d491/Makefile#L632-L636Signed-off-by: you06 <you1474600@gmail.com> (commit: d457c85)