Tests

Add better SDESystem error when it hasn't been `structural_simplify` #1042

Job	Run time
Tests (1, Downstream) / Tests - Downstream	21m 41s
Tests (1, Extended) / Tests - Extended	17m 9s
Tests (1, Extensions) / Tests - Extensions	33m 5s
Tests (1, Initialization) / Tests - Initialization	30m 2s
Tests (1, InterfaceI) / Tests - InterfaceI	35m 15s
Tests (1, InterfaceII) / Tests - InterfaceII	30m 12s
Tests (1, RegressionI) / Tests - RegressionI	14m 18s
Tests (1, SymbolicIndexingInterface) / Tests - SymbolicIndexingInterface	17m 56s
Tests (lts, Downstream) / Tests - Downstream	10m 53s
Tests (lts, Extended) / Tests - Extended	7m 45s
Tests (lts, Extensions) / Tests - Extensions	19m 48s
Tests (lts, Initialization) / Tests - Initialization	17m 54s
Tests (lts, InterfaceI) / Tests - InterfaceI	23m 55s
Tests (lts, InterfaceII) / Tests - InterfaceII	17m 23s
Tests (lts, RegressionI) / Tests - RegressionI	7m 26s
Tests (lts, SymbolicIndexingInterface) / Tests - SymbolicIndexingInterface	8m 37s
Tests (pre, Downstream) / Tests - Downstream	22m 6s
Tests (pre, Extended) / Tests - Extended	17m 56s
Tests (pre, Extensions) / Tests - Extensions	32m 54s
Tests (pre, Initialization) / Tests - Initialization	29m 1s
Tests (pre, InterfaceI) / Tests - InterfaceI	36m 47s
Tests (pre, InterfaceII) / Tests - InterfaceII	30m 10s
Tests (pre, RegressionI) / Tests - RegressionI	14m 5s
Tests (pre, SymbolicIndexingInterface) / Tests - SymbolicIndexingInterface	18m 23s
	8h 34m 41s

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