Add ASL test for externally-defined objects

This doesn't actually appear to emit externals, assuming we're decoding
correctly. I think the compiler is actually manipulating this into a scope
definition, and then assuming the object is correctly loaded. Fine.

tests/external.asl

@@ -0,0 +1,10 @@
+DefinitionBlock("external.aml", "DSDT", 1, "RSACPI", "EXTRL", 1) {
+	Scope(_SB) {
+		External(FOO, MethodObj, IntObj, {IntObj})
+	}
+
+	Name(X, Zero)
+	Method(BAR) {
+		Store(\_SB.FOO(4), X)
+	}
+}

tests/power_res.asl

@@ -14,4 +14,3 @@ DefinitionBlock("power_res.aml", "DSDT", 1, "RSACPI", "PWRRES", 1) {
 		}
 	}
 }
-