[PubgrubTests] append, not assign, packageDependencies to value in dictionary keyed by product

`dependencies` passed to `DependencyGraphBuilder.serve` can have duplicate product keys, so when `dependencies` are iterated, the same `product` can appear more than once, each time with possibly different `filteredDependencies`. Assigning `packageDependencies` mapped from a `product`'s `filteredDependencies` to the value in a different dictionary keyed by the same `product` overrides any existing value from a previous assignment. Appending `packageDependencies` instead preserves all previous values.

Author: WowbaggersLiquidLunch

Tests/PackageGraphTests/PubgrubTests.swift

@@ -2193,7 +2193,7 @@ class DependencyGraphBuilder {
             let packageDependencies: [MockContainer.Dependency] = filteredDependencies.map {
                 (container: reference(for: $0), requirement: $1.0, products: $1.1)
             }
-            container.dependencies[version.description, default: [:]][product] = packageDependencies
+            container.dependencies[version.description, default: [:]][product, default: []] += packageDependencies
         }
         self.containers[package] = container
     }