Add a test which is using namespaced packages

See #163

Author: maoberlehner

test/files/node_modules/@namespace/test-package-css/css/index.css

@@ -6,3 +6,12 @@
 
 .test-package-css {
   content: 'css index file'; }
+
+.namespace-test-package-main {
+  content: 'namespace main scss file'; }
+
+.namespace-test-package-main {
+  content: 'namespace main css file'; }
+
+.namespace-test-package-css {
+  content: 'namespace css index file'; }

test/files/node_modules/@namespace/test-package-css/package.json

@@ -1,3 +1,7 @@
 @import '~test-package-main';
 @import '~test-package-main/css/main';
 @import '~test-package-css';
+
+@import '~@namespace/test-package-main';
+@import '~@namespace/test-package-main/css/main';
+@import '~@namespace/test-package-css';

test/files/node_modules/@namespace/test-package-main/css/main.css

@@ -6,7 +6,7 @@ import { exec } from 'child_process';
 import * as packageImporter from '../packages/node-sass-package-importer/dist/index';
 
 describe(`packageImporter()`, () => {
-  test(`It should import the modules main file.`, () => {
+  test(`It should import files from modules.`, () => {
     const expectedResult = fs.readFileSync(`test/files/package-import.css`, {
       encoding: `utf8`,
     });
@@ -20,7 +20,7 @@ describe(`packageImporter()`, () => {
     expect(result).toBe(expectedResult);
   });
 
-  test(`It should import the modules main file via CLI.`, async () => {
+  test(`It should import files from modules via CLI.`, async () => {
     // tslint:disable-next-line max-line-length
     const cmd = `cd ${__dirname}/files && node ../../node_modules/.bin/node-sass --importer ../../packages/node-sass-package-importer/dist/cli.js package-import.scss`;
     const expectedResult = fs.readFileSync(`test/files/package-import.css`, {