Tests

Move Statistics.jl to weak dependency extension #217

Job	Run time
Tests (pre, Downstream) / Tests - Downstream	17m 27s
Tests (1, Core) / Tests - Core	2m 6s
Tests (pre, Core) / Tests - Core	1m 39s
Tests (lts, Core) / Tests - Core	1m 53s
Tests (1, Downstream) / Tests - Downstream	9m 48s
Tests (lts, Downstream) / Tests - Downstream	9m 54s
	42m 47s

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