Add test for load command

There doesn't seem to by any test for the load command.  I suggest to
add this test, so that we can have a minimum of confidence we don't
break it completely while refactoring the code that implements it.

gdb/testsuite/ChangeLog:

	* gdb.base/load-command.c: New file.
	* gdb.base/load-command.exp: New file.
	* lib/gdb.exp (gdb_is_target_remote_prompt): Rename to...
	(gdb_is_target_1): ...this, and generalize for other targets
	than just remote.
	(gdb_is_target_remote): Use gdb_is_target_1.
	(gdb_is_target_native): use gdb_is_target_1.