test(html): remove PROBE_DS debug scaffolding from single-fixture test

[crowlbot]

The html_doc_files_single test reads a PROBE_DS environment variable to
override the fixture directory:

get_files(std::env::var("PROBE_DS").as_deref().unwrap_or("single")).await,

This is leftover debugging scaffolding — the test should always run against the
single fixture. The env-var indirection has no purpose in the committed test
and silently lets an external variable redirect it to a different fixture.

Restore the test to call get_files("single") directly.

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