use mock s3 for tests

[axrj]

• Fixes currently failing tests by using s3 mock since access to S3 bucket is restricted for security reasons
• Adds variable to skip s3 tests for node 16. This should be investigated and fixed later with #638
• Upgrade few dependencies especially node-fetch which fixes a security vuln #633
• Removed proxy.test.js since the file is a near duplicate of build.test.js but using a proxy to access s3 for external forks which is now not required since tests use mocked s3
• Adds node-webkit test job back as it was disabled in #614