[3.13] gh-119791: Fix new Tkinter tests for wantobjects=0 (GH-119792) (GH-119794)

PhotoImage.get() retruns a string instead of a 3-tuple of integers
in this case.
(cherry picked from commit e875c2d752fed0a8d16958dc7b331e66a2476247)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>