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.