Test framework: Return fully typed TestPackageManager from TestDub

Using covariant return type, we can avoid the need to change the base class,
and leave all the overloading in the child class.

source/dub/packagemanager.d

@@ -243,21 +243,6 @@ class PackageManager {
 		return this.lookup(name, ver);
 	}
 
-    /**
-     * Adds a `Package` to this `PackageManager`
-     *
-     * This is currently only available in unittests as it is a convenience
-     * function used by `TestDub`, but could be generalized once IO has been
-     * abstracted away from this class.
-     */
-	version (unittest) Package add(Package pkg)
-	{
-		// See `PackageManager.addPackages` for inspiration.
-		assert(!pkg.subPackages.length, "Subpackages are not yet supported");
-		this.m_internal.fromPath ~= pkg;
-		return pkg;
-	}
-
 	/// ditto
 	deprecated("Use the overload that accepts a `Version` as second argument")
 	Package getPackage(string name, string ver, bool enable_overrides = true)

source/dub/test/base.d

@@ -82,6 +82,17 @@ public class TestDub : Dub
 	/// Reintroduce parent overloads
 	public alias loadPackage = Dub.loadPackage;
 
+	/**
+	 * Returns a fully typed `TestPackageManager`
+	 *
+	 * This exposes the fully typed `PackageManager`, so that client
+	 * can call convenience functions on it directly.
+	 */
+	public override @property inout(TestPackageManager) packageManager() inout
+	{
+		return cast(inout(TestPackageManager)) this.m_packageManager;
+	}
+
 	/**
 	 * Creates a package with the provided recipe
 	 *
@@ -212,6 +223,21 @@ package class TestPackageManager : PackageManager
 
 		return null;
     }
+
+    /**
+     * Adds a `Package` to this `PackageManager`
+     *
+     * This is currently only available in unittests as it is a convenience
+     * function used by `TestDub`, but could be generalized once IO has been
+     * abstracted away from this class.
+     */
+	public Package add(Package pkg)
+	{
+		// See `PackageManager.addPackages` for inspiration.
+		assert(!pkg.subPackages.length, "Subpackages are not yet supported");
+		this.m_internal.fromPath ~= pkg;
+		return pkg;
+	}
 }
 
 /**