Blueprints: add unit tests to the mkdir step

[reimic]

What does this PR do?

Adds tests to the mkdir step.

Tests directory creation minding idempotence and recursivity.

Part of the #756 effort.

[adamziel]

I am slightly surprised this step allows for creation of directories with periods in their name.

Periods are valid characters in directory names, see this StackOverflow reply about forbidden characters.

Also, if you could confirm my understanding: this step never fails - it will either do what it is told to do, or do nothing (if the requested directory already exists).

I can't remember off the top of my head, but if it doesn't throw an exception when the directory already exists, then you're right.

[adamziel]

@reimic looks good, let's clean up the PR description and I'm happy to merge.

[adamziel]

Thank you @reimic !