**Backport:** #7715
- Replaces `github.com/go-testfixtures/testfixtures` with a homebrew solution that is fully compatible.
- The reason to replace this library is that it pulls in a lot of other libraries which is causing issues: (1) the test binary becomes bigger than necessary which really shows in incremental build times (this patch removes 27.6MiB of the integration test binary) (2) it pulls in libraries (mainly database drivers) that are not used and are not easy to upgrade in case of a security vulnerability, causing CI failures.
(cherry picked from commit 32e64ccd34)
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7729
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>
