Allow Skipping One Unit Test

[oleibman]

Alone in the test suite, URLImageTest needs to access the internet. It's a little fragile (the site that it's looking for may go away or change), but no real problem. However, on my system, it runs afoul of my proxy. Rather than jumping through hoops when I run the test suite (which happens very often), I am changing the test to skip if an environment variable is set to a specific value. This should not adversely affect anyone, and the test will still run in github, but it will help me a lot.

This is:

- [ ] a bugfix
- [ ] a new feature
- [x] unit testing in private environment 

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?