Test: Add a missing test case to derivative_symbols.swift.

Author: tshortli

test/AutoDiff/TBD/derivative_symbols.swift

@@ -80,6 +80,16 @@ public struct Struct: Differentiable {
   }
 }
 
+extension Struct {
+  // Test a derivative declared in an extension.
+  @derivative(of: init(_:))
+  public static func _vjpInit(_ value: Float)
+  -> (value: Struct, pullback: (TangentVector) -> (Float.TangentVector)) {
+    fatalError()
+  }
+}
+
+
 extension Array where Element == Struct {
   @differentiable(reverse)
   public func sum() -> Float {