test(rewrite): systematic coverage matrix for assertion rewriting

The rewriter is tested through its failure messages, one regression test
per issue.  That makes it hard to say whether a change improved anything:
there is no place that records what the rewriter cannot do yet.

Add a matrix over expression types with four axes -- introspection depth,
semantic equivalence, single evaluation, and evaluation order -- and
record every known gap as a strict xfail tagged with a group name.  A
change that closes one names the group and flips its markers; strictness
means it cannot forget.

The evaluation-order axis is new.  Comparing pass/fail is not enough: an
operand read after a later walrus operator rebound its name runs exactly
once and can still fail the assertion, having compared the wrong value.
assert_evaluation_order() compares what check() returns, because the
fragile operand is a bare name -- wrapping it in a call to observe it
would hoist it and hide the bug.

Behaviour is unchanged; this only describes it.

Author: RonnyPfannschmidt

changelog/14813.contrib.rst

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+Added a systematic coverage matrix for assertion rewriting in ``testing/test_assertrewrite_coverage.py``, checking introspection depth, semantic equivalence, single evaluation and evaluation order across expression types. Known gaps are recorded as strict xfails tagged with a group name, so a change can state which group it closes.