Adjust command completion output when TUI is disabled
    
    The history-scrolling commands "+", "-", "<" and ">" are only known to
    GDB when TUI is enabled.  This means the "complete pipe " command
    produces different output depending on whether TUI is present, which
    in turn caused
    
    FAIL: gdb.base/shell.exp: cmd complete "pipe "
    
    This patch provides different patterns for that test depending on
    whether or not TUI is available.
    
    2020-06-23  Sandra Loosemore  <sandra@codesourcery.com>
    
    	gdb/testsuite/
    	* lib/completion-support.exp (test_gdb_completion_offers_commands):
    	Adjust for omitted commands when TUI is disabled.