Let integration tests run in parallel (#2076) #6386

Job	Run time
Tests (default)	4m 24s
Verify MSRV	3m 5s
build_with_no_default_features (ubuntu-latest)	35s
build_with_no_default_features (macos-latest)	50s
build_with_no_default_features (windows-latest)	1m 58s
build (ubuntu-latest)	2m 4s
check (macos-latest)	8m 8s
Tests (doc)	1m 23s
check (ubuntu-latest)	6m 10s
build (windows-latest)	5m 23s
build (macos-latest)	7m 36s
	41m 36s

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