ci: test with `1`, `lts` and `pre` versions of julia #40

Job	Run time
Tests (1, Core) / Tests - Core	9m 3s
Tests (1, Downstream) / Tests - Downstream	19m 35s
Tests (lts, Core) / Tests - Core	8m 51s
Tests (lts, Downstream) / Tests - Downstream	11m 9s
Tests (pre, Downstream) / Tests - Downstream	16m 13s
Tests (pre, Core) / Tests - Core	9m 15s
	1h 14m 6s

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