libgui: Add a test server utility for unit tests.

This is being added to make it possible to test linkToDeath callbacks,
but may serve to be generally useful.

This is a complicated little thing. But it has to be this way.

Most of the complication comes from the fact that we want ephemeral
Binder services that we can kill at will on a per-test basis. Binder
doesn't let you do any binder stuff after you've forked, however, so we
needed an intermediary, non-binder process to kick off the binder
processes that actually run the test server.

The way this works is:

- libgui_test defaults to being a normal test, and intializes a
  `TestServerHost` via fork/exec of the libgui_test binary
- The first fork runs TestServerHostMain and waits on a pipe for
  commands to create servers, at which point it will fork/exec
- Each forked server creates a new ITestService service, which the
  original test connects to.

BYPASS_IGBP_IGBC_API_REASON=warren buffers

Bug: 340933794
Flag: EXEMPT test
Test: test library
Change-Id: I37ee51b13a895a8b3387b7f2db92e5069158ff46