Full migration to idiomatic pytest #1044

Job	Run time
bugwarrior-test (3.14)	36s
bugwarrior-test (3.13)	35s
bugwarrior-test (3.11)	49s
bugwarrior-development-pip-test	51s
bugwarrior-test (3.12)	39s
bugwarrior-multiarch-test (ppc64le)	10m 29s
	13m 59s

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