test: refactor tests to use `AshPostgres` #298

Job	Run time
ash-ci / audit	16s
ash-ci / mix igniter.upgrade	7s
ash-ci / MIX_ENV=test mix.compile	45s
ash-ci / mix spark.formatter --check	0s
ash-ci / mix ash.codegen --check	0s
ash-ci / mix spark.cheat_sheets --check	0s
ash-ci / MIX_ENV=dev mix.compile	0s
ash-ci / mix dialyzer	0s
ash-ci / mix format --check-formatted	0s
ash-ci / format('NIF {0} - {1} ({2})', matrix.nif, matrix.job.target, matrix.job.os)	0s
ash-ci / mix sobelow	0s
ash-ci / mix deps.unlock --check-unused	0s
ash-ci / mix credo --strict	0s
ash-ci / mix test	0s
ash-ci / mix docs	0s
ash-ci / Release	0s
ash-ci / Deploy docs to GitHub pages.	0s
	1m 8s

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