Fix relative redirect (#71932)
# Fixing a bug
## What?
Changed the formation of the redirect url to the correct way. _It’s
interesting that you had tests for this, but since only the main page
was tested, it wasn’t caught._
Expanded the tests, made a separate method to assign location, and
described it.
```js
new URL('./relative', 'https://example.com/subdir').href // 'https://example.com/relative'
new URL('./relative', 'https://example.com/subdir/').href // 'https://example.com/subdir/relative'
```
Fixes #71906 [#65893, #67966]
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Co-authored-by: Zack Tanner <1939140+ztanner@users.noreply.github.com>
