acceptance: forbid empty test.toml files (databricks#5297)

Fail any acceptance test whose `test.toml` parses to an empty table
(zero-byte or comment-only). The check lives in the acceptance test
runner's config loader. Deletes 20 pre-existing offenders -- 4 zero-byte
placeholders and 16 comment-only files. None of the comments in those
files are relevant for the test.

This codifies a review comment that shows up from time to time and
thereby removes ambiguity/overhead.

This pull request and its description were written by Isaac.

Author: pietern