Fix for demo

[Hazhzeng]

The issue we were facing was reported here jestjs/jest#5064
This is a jest issue. PowerShell will interpreted all outputs in stderr as failure. Thus, I change that to use bash.

Before merging this PR please do the following steps:

1. Acquire the HelloWorldSlots function app stage slot SCM credentials and placed it in GitHub repository secret SCM_CREDENTIALS.
2. In the HelloWorldSlots function app stage slot app settings, add WEBSITE_RUN_FROM_PACKAGE = 1 manually, (since SCM credentials do not have privilege to change app settings)

There's a limitation in slot deployment using slot-name with SCM for WebApp and FunctionApp GitHub Actions. The related issues are here, Azure/webapps-deploy#6, Azure/functions-action#25