ci: add nightly contract tests with long-running tests and Slack notification #347

Job	Run time
test-packaging	4m 43s
test-contract-tests	4m 47s
test-platforms-and-versions (ubuntu-latest, 17)	3m 36s
test-platforms-and-versions (ubuntu-latest, 19)	3m 37s
test-platforms-and-versions (ubuntu-latest, 8)	3m 36s
test-platforms-and-versions (ubuntu-latest, 11)	3m 41s
	24m 0s

Provide feedback