bpo-44336: Prevent tests hanging on child process handles on Windows (GH-26578)

bpo-44336: Prevent tests hanging on child process handles on Windows (GH-26578)

Replace the child process `typeperf.exe` with a daemon thread that reads the performance counters directly.  This prevents the issues that arise from inherited handles in grandchild processes (see issue37531 for discussion).

We only use the load tracker when running tests in multiprocess mode. This prevents inadvertent interactions with tests expecting a single threaded environment.  Displaying load is really only helpful for buildbots running in multiprocess mode anyway.