Adds tests for the interactive profile browser started by python -m pstats, which had none (test_pstats.py only covered the Stats API).

The browser only exists inside the module's __main__ block, so the tests run it in a subprocess with script_helper.spawn_python() and feed commands on stdin. A small profile is generated once per class with cProfile from two module-level functions, one of which calls the other three times, so the expected call counts, callers and callees are known.

Covered:

• Startup banner, prompt, quit, and EOF on stdin.
• A profile given on the command line, several profiles (added together), and a missing one.
• read (with an argument, without one to reload, missing file, non-marshal file) and add (including a missing file).
• stats with an integer, a fraction, a regular expression, and an out-of-range fraction.
• sort by a full key and by a unique prefix, sort with no or an invalid key, reverse, strip.
• callers and callees.
• help and help stats.
• Every command's behaviour when no statistics are loaded.

Assertions avoid anything timing-dependent: they check call counts, the relative order of the two known functions, and the exact messages printed by the browser. The class is guarded with support.requires_subprocess(). The 20 new tests take about 1.2 s on a debug build here.

Ran test_pstats repeatedly, make patchcheck and the pre-commit hooks on macOS.

• Issue: Add tests for command-line interface #131178