tests: use %TESTNUMBER instead of fixed number

This makes the tests easier to copy and relocate to other test numbers
without having to update content.

Closes #6738

httpauth: make multi-request auth work with custom port

When doing HTTP authentication and a port number set with CURLOPT_PORT,
the code would previously have the URL's port number overr

httpauth: make multi-request auth work with custom port

When doing HTTP authentication and a port number set with CURLOPT_PORT,
the code would previously have the URL's port number override as if it
had been a redirect to an absolute URL.

Added test 1568 to verify.

Reported-by: UrsusArctos on github
Fixes #6397
Closes #6400

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