Added test utility functions: importskip(), serverskip()

[jesper-friis]

Description

Added test utility function serverskip() to skip a test if a needed server is down.
Also added importskip() for convenience.

Motivation for this PR: Avoid that tests fail when services like play.min.io are down.

Type of change

• Bug fix & code cleanup
• New feature
• Documentation update
• Test update

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[francescalb]

Looks OK, but I think this will easily end up resulting in tests not running when we should not miss that they are failing. I think this PR might have unintened repercussions.