[BUG] Usage docs do not test for library name prefix on test files

[hdwalters]

Describe the bug
As part of the standard library rename in pull request #639, we renamed the integration test files to match the new standard library function names, and added the library name as a prefix if missing. For example, function text_contains is tested in file text_contains.ab, but replace is tested in text_replace.ab. However, when we run amber docs --usage, we find all test files matching replace*.ab, but not text_replace*.ab.

To Reproduce
Run amber docs --usage src/std/text.ab.

Expected behavior

• The usage docs should reference the prefixed as well as the unprefixed test files.
• While I'm at it, I would also like to remove the excess blank lines from the generated Markdown files, as we inconsistently use one, two or three newlines, depending on context.
• Finally, I would like to make amber docs text.ab write to standard output if the specified output directory is -; this will make testing changes simpler.

Additional context
N/A