introduce relative_filename and use it

introduce relative_filename and use it

This introduces a new relative_filename proc to gdb.exp and changes
some tests to use it.  This helps make these tests parallel-safe.

2013-11-04  Tom Tromey  <tromey@redhat.com>

	* gdb.base/fullname.exp: Use standard_output_file,
	relative_filename.
	* gdb.base/hashline1.exp: Use standard_testfile,
	standard_output_file, relative_filename, clean_restart.
	* gdb.base/hashline2.exp: Use standard_testfile,
	standard_output_file.
	* gdb.base/hashline3.exp: Use standard_testfile,
	standard_output_file, relative_filename.
	* lib/gdb.exp (relative_filename): New proc.