Backport of "tests: refactor `sharedtest.ParsedAccConfig`" to rel/v0.20 #1832

Job	Run time
build-test (tofu)	1m 41s
omicron-version	9s
build-test (terraform)	2m 51s
publish-image	6s
acceptance / acceptance (terraform)	6m 27s
acceptance / acceptance (tofu)	5m 55s
	17m 9s

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