gdb/testsuite/tui: Split enter_tui into two procs

gdb/testsuite/tui: Split enter_tui into two procs

Split Term::enter_tui into two procedures, a core which does the
setup, but doesn't actually enable tui mode, and the old enter_tui
that calls the new core, and then enables tui mode.

This is going to be useful in a later commit.

gdb/testsuite/ChangeLog:

	* lib/tuiterm.exp (Term::prepare_for_tui): New proc.
	(Term::enter_tui): Use Term::prepare_for_tui.

Change-Id: I501dfb2ddaa4a4e7246a5ad319ab428e4f42b3af