tests: add tests for the current state of "Checking a Local Folder with URL Remapping"

[katrinafyi]

this adds tests for the use case of "Checking a Local Folder with URL Remapping", as motivated in lycheeverse/lycheeverse.github.io#140. it makes no behaviour changes.

to review this PR, i suggest first understanding the use-case, then you can read the input files and expected URLs to check that they make sense. there are two new .md files inside the fixtures folder, one for whole domain mapping and one for subpath mapping. the markdown text explains the reasoning for each URL.

the remaps are deeply unpleasant and you shouldn't try too hard to understand them. they're also different from lycheeverse/lycheeverse.github.io#140 because i found new edge cases and new bugs. i'll update the docs pr after...

issues arising:

• Relative parent links can traverse outside of root-dir #1953
• root-dir incorrectly applies to links in remote HTML files #1964