Add --rerun-show-tracebacks to surface retried failures

[vokracko]

Currently pytest-rerunfailures only emits the traceback for the final attempt of a flaky test. If the last attempt passes, the failures that triggered the reruns are silently discarded; if the last attempt fails, the earlier tracebacks are gone even when they hit a different exception. That makes triaging flaky tests harder than it needs to be — you can see that a test was retried, but not what actually went wrong on each attempt.

This adds --rerun-show-tracebacks, which appends each retried attempt's traceback to the rerun test summary info section. The section is emitted automatically when the flag is set, so -rR is no longer required.

For example, a test that hits a transient assertion twice and passes on the third attempt:

test_demo.py::test_eventually_passes RERUN                               [100%]
test_demo.py::test_eventually_passes RERUN                               [100%]
test_demo.py::test_eventually_passes PASSED                              [100%]
=========================== rerun test summary info ============================
RERUN test_demo.py::test_eventually_passes
>       assert attempt > 2, f"transient failure on attempt {attempt}"
E       AssertionError: transient failure on attempt 1
E       assert 1 > 2
RERUN test_demo.py::test_eventually_passes
>       assert attempt > 2, f"transient failure on attempt {attempt}"
E       AssertionError: transient failure on attempt 2
E       assert 2 > 2

As a side effect, the per-char loop in pytest_terminal_summary is gone. The loop called show_rerun once per matching char in reportchars, so passing -r rR doubled every RERUN line in the summary; the new code calls show_rerun once. Happy to revert that part if the duplication is intentional.

Fixes #156.

[icemac]

LGTM.