Use `test_timeout' consistently across protocols

The `test_timeout' global parameter was added with commit 73db65f53795,
however for the `unix' protocol only and then only in the local case.
Update the remote case of the `unix' protocol, other protocols that use
various timeout sources, and the `qemu' board to take any `test_timeout'
setting into account as appropriate.  Amend the manual accordingly.

	* baseboards/qemu.exp (qemu_load): Respect global `test_timeout'
	setting.
	* config/gdb-comm.exp (gdb_comm_load): Likewise.
	* config/gdb_stub.exp (gdb_stub_load): Likewise.
	* config/sim.exp (sim_load): Likewise.
	* config/unix.exp (unix_load): Likewise, in the remote case.
	* doc/dejagnu.texi (Local configuration file): Update
	accordingly.