test(context): Introduce an explicit value merging test

Introduces a test which will merge variables defined at every possible
layer together and ensure that the loaded context configuration is as
expected.

The test data provides an actual resource set template that can be
tested locally from a kontemplate source checkout:

    kontemplate template context/testdata/merging/context.yaml --var cliVar=cliVar

context/context_test.go

@@ -293,3 +293,25 @@ func TestSetInvalidVariablesFromArguments(t *testing.T) {
 		t.Error("Expected invalid variable to return an error")
 	}
 }
+
+// This test ensures that variables are merged in the correct order.
+// Please consult the test data in `testdata/merging`.
+func TestValueMergePrecedence(t *testing.T) {
+	cliVars:= []string{"cliVar=cliVar"}
+	ctx, _ := LoadContext("testdata/merging/context.yaml", &cliVars)
+
+	expected := map[string]interface{}{
+		"defaultVar": "defaultVar",
+		"importVar": "importVar",
+		"globalVar": "globalVar",
+		"includeVar": "includeVar",
+		"cliVar": "cliVar",
+	}
+
+	result := ctx.ResourceSets[0].Values
+
+	if !reflect.DeepEqual(expected, result) {
+		t.Errorf("Merged values did not match expected result: \n%v", result)
+		t.Fail()
+	}
+}