testlibrary: Let the test web server's stderr go to the test's stderr

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1760
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f