ci: add nightly contract tests with long-running tests and Slack notification #346

Job	Run time
test-platforms-and-versions (ubuntu-latest, 17)	3m 35s
test-platforms-and-versions (ubuntu-latest, 19)	3m 41s
test-platforms-and-versions (ubuntu-latest, 8)	3m 31s
test-contract-tests	3m 57s
test-packaging	4m 46s
test-platforms-and-versions (ubuntu-latest, 11)	3m 39s
	23m 9s

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