t0052: add simple-ipc tests and t/helper/test-simple-ipc tool

t0052: add simple-ipc tests and t/helper/test-simple-ipc tool

Create t0052-simple-ipc.sh with unit tests for the "simple-ipc" mechanism.

Create t/helper/test-simple-ipc test tool to exercise the "simple-ipc"
functions.

When the tool is invoked with "run-daemon", it runs a server to listen
for "simple-ipc" connections on a test socket or named pipe and
responds to a set of commands to exercise/stress the communication
setup.

When the tool is invoked with "start-daemon", it spawns a "run-daemon"
command in the background and waits for the server to become ready
before exiting.  (This helps make unit tests in t0052 more predictable
and avoids the need for arbitrary sleeps in the test script.)

The tool also has a series of client "send" commands to send commands
and data to a server instance.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>