[3.15] gh-151678: Add tests for tkinter font, image, variable, Misc and Wm methods...

[3.15] gh-151678: Add tests for tkinter font, image, variable, Misc and Wm methods (GH-151751) (GH-151754)

* font: copy(), the config alias, the displayof argument of measure and
  metrics, and the errors raised for invalid options and a wrong number
  of arguments;
* image: the cget method and the config alias, and the errors raised by
  transparency_get and transparency_set;
* variable: that initialize is an alias of set and the deprecated trace
  is an alias of trace_variable;
* Misc: tk_focusNext, tk_focusPrev, tk_strictMotif, tk_bisque and
  option_readfile;
* Wm: wm_iconphoto.
(cherry picked from commit 66cc04855100c3865bd01adfe92a3a02dbc3a914)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>