dtc: Testcase for /include/ directive

This patch adds a testcase for the /include/ directive.  It assembles
a sample dts file with many /include/ directives at a variety of
different lexical / grammatical contexts.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/include7.dts

@@ -0,0 +1,9 @@
+	subnode@1 {
+		compatible = "subnode1";
+		prop-int = [deadbeef];
+
+		subsubnode {
+			compatible = "subsubnode1", "subsubnode";
+			prop-int = <0xdeadbeef>;
+		};
+	};