backend/ftp: increase testUploadTimeout.maxTime to 10 seconds

On slow machines (e.g. Github CI), especially if GOARCH=386,
the test for cmd/serve/ftp could fail if this value is too small.

Fixes #5783
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