log: suppressing logs with -logtostderr=false no longer works in benchmarks

As of #57134 passing `-logtostderr=false` as a TESTFLAG in benchmarks errs: `flag provided but not defined: -logtostderr`. This flag was useful for suppressing logs from stdout (I'm confused why the flag was named `-logtostderr`) while running benchmarks so that only the benchmark output was written.

cc @knz 