Hard-code tests that used FiniteDifferences

Author: bencottier

test/rules.jl

@@ -129,9 +129,11 @@ _second(t) = Base.tuple_type_head(Base.tuple_type_tail(t))
         Ω = complex_times(x)
         Ω_fwd, Ω̇ = frule((nothing, ẋ), complex_times, x)
         @test Ω_fwd == Ω
+        @test Ω̇ ≈ (1 + 2im) * ẋ
         Ω_rev, back = rrule(complex_times, x)
         @test Ω_rev == Ω
         ∂self, ∂x = back(Ω̄)
         @test ∂self == NO_FIELDS
+        @test ∂x ≈ (1 - 2im) * Ω̄
     end
 end