Refactor ModelAdmin breadcrumbs regression tests to be more flexible

[thibaudcolas]

Attempts to fix test regressions in stable/3.0.x by making the ModelAdmin breadcrumb tests more flexible. We’ve changed those templates in #8491, and only cherry-picked the most fundamental changes to stable/3.0.x, which is causing test failures for that branch only.

There are 3 "breadcrumbs position in ModelAdmin" test cases, 2 were failing. I’ve updated those two to work the same as the one that was still passing. I’ve also updated the three tests to check the presence of the nav landmark region, rather than the ul, as that’s more semantically what should denote where the breadcrumbs are.

I find the value of this "position" test a bit dubious compared to just testing whether the breadcrumbs are present or not. Hopefully something we could revisit as part of #8539.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://thibaudcolasbug8471-modeladmin-nvurz.squash.io

[gasman]

With this change there are now failures against both main and stable/3.0.x:

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ERROR: test_choose_inspect_page (wagtail.contrib.modeladmin.tests.test_page_modeladmin.TestHeaderBreadcrumbs)
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Traceback (most recent call last):
  File "/Users/matthew/Development/tbx/wagtail/devscript/wagtail/wagtail/contrib/modeladmin/tests/test_page_modeladmin.py", line 406, in test_choose_inspect_page
    position_of_breadcrumbs = content_str.index('<nav aria-label="Breadcrumb">')
ValueError: substring not found

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ERROR: test_choose_parent_page (wagtail.contrib.modeladmin.tests.test_page_modeladmin.TestHeaderBreadcrumbs)
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Traceback (most recent call last):
  File "/Users/matthew/Development/tbx/wagtail/devscript/wagtail/wagtail/contrib/modeladmin/tests/test_page_modeladmin.py", line 377, in test_choose_parent_page
    position_of_breadcrumbs = content_str.index('<nav aria-label="Breadcrumb">')
ValueError: substring not found

======================================================================
ERROR: test_choose_inspect_model (wagtail.contrib.modeladmin.tests.test_simple_modeladmin.TestHeaderBreadcrumbs)
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Traceback (most recent call last):
  File "/Users/matthew/Development/tbx/wagtail/devscript/wagtail/wagtail/contrib/modeladmin/tests/test_simple_modeladmin.py", line 952, in test_choose_inspect_model
    position_of_breadcrumbs = content_str.index('<nav aria-label="Breadcrumb">')
ValueError: substring not found

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Ran 107 tests in 9.304s

[gasman]

Superseded by #8549