Add `WagtailTestUtils.get_soup()` method to get a `BeautifulSoup` object #120

Job	Run time
test-mysql (3.8, Django>=3.2,<3.3, false)	23m 17s
test-mysql (3.10, Django>=4.2,<4.3, false, --parallel)	14m 55s
test-postgres (3.8, Django>=3.2,<3.3, false, --parallel)	16m 22s
test-postgres (3.11, Django>=4.2,<4.3, postgres:12, notz, false, --parallel)	12m 54s
test-postgres (3.10, Django>=4.1,<4.2, false, emailuser, --parallel)	11m 33s
test-postgres (3.10, git+https://github.com/django/django.git@stable/4.2.x#egg=Django, true, post...	23m 32s
test-postgres (3.10, git+https://github.com/django/django.git@main#egg=Django, true, postgres:12,...	11m 32s
test-postgres-elasticsearch7 (3.8, Django>=4.1,<4.2, false)	12m 14s
test-postgres-elasticsearch6 (3.8, Django>=3.2,<3.3)	9m 4s
test-sqlite (3.9, Django>=4.1,<4.2)	10m 52s
test-sqlite-elasticsearch5 (3.8, Django>=3.2,<3.3)	8m 27s
test-sqlite-elasticsearch8 (3.10, Django>=4.2,<4.3, emailuser)	7m 3s
coverage	1m 0s
	2h 42m 45s

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