Added notice to all examples for using the test server. Use test serv…

[PGijsbers]

Uses the #680 functionality and adds warning messages that the test server is used for examples.

• This can't be merged before Easy access test server #680.
• We should go back and rework examples to use the live server if they do not upload anything.

Unfortunately I probably won't have the time to finish this before the weekend (and I won't have time to work on it then, either). Feel free to take over this PR.

[PGijsbers]

This was intended to be a draft >.> not sure what happened there

[codecov-io]

Codecov Report

Merging #684 into develop will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #684 +/- ##
========================================
Coverage 90.76% 90.76% ========================================
Files 36 36 Lines 3650 3650 ========================================
Hits 3313 3313 Misses 337 337

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[mfeurer]

@PGijsbers I added your suggestions. Could you please check the changes I made?

[PGijsbers]

Looks good. I'm merging this so that we can then update examples with individual PRs as necessary.