simple test suite fix in gdb.mi

simple test suite fix in gdb.mi

This is another simple test suite change for the parallelization
project.

This changes mi-basics.exp to avoid the use of subdir and objdir and
instead use standard_output_file.

There are still some uses of objdir, but as noted in a new comment in
the patch, these uses are parallel-safe.

	* gdb.mi/mi-basics.exp: Use standard_output_file.
	(test_dir_specification, test_cwd_specification)
	(test_path_specification): Use testsubdir, not subdir and objdir.