Are you running on Linux or Mac?  What tk version?

About the 'invalid command name' message: PR3622, patching idlelib.codecontext, merged to master 13 hours ago, fixed this for me on Windows.  (It added CodeContext.__del__.)  Did it fail on your machine or was it not yet merged on your machine?

About the test failure.  Hs is a Text with about 14 lines.  The test initially executes
        hs.see(1.0)
        hs.update_idletasks()
hs.bbox(start) means that the tag start is not visible,  which should realistically not be possible.  But a quick web search suggests there have been problems with HiDPI and tk, and hence tkinter.

I am curious what value of start fails.  Could you run once with 'print(start)' added above bbox(start)?

Does inserting the following just before hs.bbox(start) fix the issue?
        hs.see(start)
        hs.update_idletasks()
If not, we can skip.  Does the following print one or multiple skip messages?
        try:
            x, y, dx, dy = hs.bbox(start)
        except TypeError:
            self.skipTest(f'bbox failure with start {start}')