tests

Add "@abstract" tag support #469

Job	Run time
Tests (8.1, ubuntu-latest, prefer-lowest)	14s
Tests (8.1, ubuntu-latest, prefer-stable)	15s
Tests (8.1, macos-latest, prefer-lowest)	1m 40s
Tests (8.1, macos-latest, prefer-stable)	1m 36s
Tests (8.1, windows-latest, prefer-lowest)	38s
Tests (8.1, windows-latest, prefer-stable)	38s
Tests (8.2, ubuntu-latest, prefer-lowest)	13s
Tests (8.2, ubuntu-latest, prefer-stable)	12s
Tests (8.2, macos-latest, prefer-lowest)	1m 25s
Tests (8.2, macos-latest, prefer-stable)	1m 9s
Tests (8.2, windows-latest, prefer-lowest)	37s
Tests (8.2, windows-latest, prefer-stable)	39s
Tests (8.3, ubuntu-latest, prefer-lowest)	18s
Tests (8.3, ubuntu-latest, prefer-stable)	22s
Tests (8.3, macos-latest, prefer-lowest)	1m 26s
Tests (8.3, macos-latest, prefer-stable)	1m 6s
Tests (8.3, windows-latest, prefer-lowest)	43s
Tests (8.3, windows-latest, prefer-stable)	38s
Tests (8.4, ubuntu-latest, prefer-lowest)	14s
Tests (8.4, ubuntu-latest, prefer-stable)	12s
Tests (8.4, macos-latest, prefer-lowest)	1m 39s
Tests (8.4, macos-latest, prefer-stable)	2m 7s
Tests (8.4, windows-latest, prefer-lowest)	37s
Tests (8.4, windows-latest, prefer-stable)	40s
	19m 18s

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