testscript: test custom conditions

[datacharmer]

Add a test for custom conditions and a brief example in the documentation.

[rudifa]

I found this PR while looking for discussions about custom conditions in testscript.

I could not find in this PR any doc that would explain its purpose and use cases; reading the proposed code changes lead me to think that its purpose is different from mine.

EDIT: I removed further comments from here, and opened an issue Feature proposal: use environment variables as custom conditions in a txtar script #241