fix: inject app.css file from unit-test-runner on test command (#949)

Currently the app.css file from unit-test-runner is not included in bundle/vendor files on test command so the application remains without app.css file.  The `tns-core-modules` shows an error in this situation with the in their latest next versions:
```
JS: Error: Could not load CSS from ./app.css: Error: com.tns.NativeScriptException: Failed to find module: "./app.css", relative to: app//
JS:     com.tns.Module.resolvePathHelper(Module.java:146)
JS:     com.tns.Module.resolvePath(Module.java:55)
JS:     com.tns.Runtime.callJSMethodNative(Native Method)
JS:     com.tns.Runtime.dispatchCallJSMethodNative(Runtime.java:1212)
JS:     com.tns.Runtime.callJSMethodImpl(Runtime.java:1092)
JS:     com.tns.Runtime.callJSMethod(Runtime.java:1079)
JS:     com.tns.Runtime.callJSMethod(Runtime.java:1059)
JS:     com.tns.Runtime.callJSMethod(Runtime.java:1051)
JS:     com.tns.NativeScriptActivity.onCreate(NativeScriptActivity.java:19)
JS:     android.app.Activity.performCreate(Activity.java:7136)
JS:     android.app.Activity.performCreate(Activity.java:7127)
JS:     android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
JS:     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
JS:     android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
JS:     android.app.servertransac...
```

So we need to inject app.css in order to prevent this error :)

Author: Fatme

unit-testing-config-loader.js

@@ -6,10 +6,13 @@ module.exports = function ({ appFullPath, projectRoot, angular, rootPagesRegExp
     const testFilesRegExp = /tests\/.*\.(ts|js)/;
     const runnerFullPath = join(projectRoot, "node_modules", "nativescript-unit-test-runner");
     const runnerRelativePath = convertSlashesInPath(relative(appFullPath, runnerFullPath));
+    const appCssFilePath = join(runnerRelativePath, "app.css");
     let source = `
         require("tns-core-modules/bundle-entry-points");
         const runnerContext = require.context("${runnerRelativePath}", true, ${rootPagesRegExp});
         global.registerWebpackModules(runnerContext);
+        global.registerModule("${appCssFilePath}", () => require("${appCssFilePath}"));
+        require("tns-core-modules/application").setCssFileName("${appCssFilePath}");
     `;
 
     if (angular) {