The procedure to find an unused port for the debuginfod tests is susceptible...

The procedure to find an unused port for the debuginfod tests is susceptible to a TOCTOU failure.  Change port finding in order to avoid this. Also use 'expect' to interact with the server process since we now use the server's output to determine whether a port is in use.

	* binutils/testsuite/binutils-all/debuginfod.exp: Improve port
	selection.