IDLE: protect macOS Tk() call when no GUI

[terryjreedy]

The no-gui (mock Text) test_squeezer.SqueezerTest.test_write_stdout indirectly calls squeezer.Squeezer.mywrite, which calls macosx.isAquaTk, which calls _init_tk_type, which calls tkinter.Tk to initialize macosx._tk_type. When a test is run on macOS with no GUI, this crashes; see #97032 for an example.

test_squeezer later calls isAquaTk directly. Even if we fix test_squeezer, there are about 10 other isAquaTk calls in idlelib that someday might be triggered by more complete tests. I will try patching _init_tk_type in branch tktype to skip Tk() when it would crash.