fix: inject app.css file from unit-test-runner on test command

[Fatme]

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 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 :)

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-dev-webpack/blob/master/CONTRIBUTING.md#testing-locally-by-running-e2e-tests
• Tests for the changes are included.

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].