Test: Remove unnecessary server_thread method
Test: Remove unnecessary server_thread method

std::thread allows any callback signature so remove
the usual pthread void * foo(void *) wrapper when starting
the server thread.

Change-Id: I3b9e705da97258933a0c6b294c2c3bbdb8f98a27
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-hwmon,openbmc/phosphor-hwmon