initialize test suite for git fetchers

solves #9388

This utilizes nixos vm tests to allow:
- writing tests for fetchTree and fetchGit involving actual networking.
- writing small independent test cases by automating local and remote repository setup per test case.

This adds:
  - a gitea module setting up a gitea server
  - a setup module that simplifies writing test cases by automating the repo setup.
  - a simple git http test case

Other improvements:
For all nixos tests, add capability of overriding the nix version to test against.
This should make it easier to prevent regressions. If a new test is added it can simply be ran against any older nix version without having to backport the test.
For example, for running the container tests against nix 2.12.0:
`nix build "$(nix eval --raw .#hydraJobs.tests.containers --impure --apply 't: (t.forNix "2.12.0").drvPath')^*" -L`

tests/nixos/fetch-git/testsupport/gitea.nix

@@ -0,0 +1,63 @@
+{ lib, nixpkgs, system, ... }: {
+  imports = [
+    ../testsupport/setup.nix
+  ];
+  nodes = {
+    gitea = { pkgs, ... }: {
+      services.gitea.enable = true;
+      services.gitea.settings.service.DISABLE_REGISTRATION = true;
+      services.gitea.settings.log.LEVEL = "Info";
+      services.gitea.settings.database.LOG_SQL = false;
+      networking.firewall.allowedTCPPorts = [ 3000 ];
+      environment.systemPackages = [ pkgs.gitea ];
+
+      # TODO: remove this after updating to nixos-23.11
+      nixpkgs.pkgs = lib.mkForce (import nixpkgs {
+        inherit system;
+        config.permittedInsecurePackages = [
+          "gitea-1.19.4"
+        ];
+      });
+    };
+    client = { pkgs, ... }: {
+      environment.systemPackages = [ pkgs.git ];
+    };
+  };
+  defaults = { pkgs, ... }: {
+    environment.systemPackages = [ pkgs.jq ];
+  };
+
+  setupScript = ''
+    import shlex
+
+    gitea.wait_for_unit("gitea.service")
+
+    gitea_admin = "test"
+    gitea_admin_password = "test123test"
+
+    gitea.succeed(f"""
+      gitea --version >&2
+      su -l gitea -c 'GITEA_WORK_DIR=/var/lib/gitea gitea admin user create \
+        --username {gitea_admin} --password {gitea_admin_password} --email test@client'
+    """)
+
+    client.wait_for_unit("multi-user.target")
+    gitea.wait_for_open_port(3000)
+
+    gitea_admin_token = gitea.succeed(f"""
+      curl --fail -X POST http://{gitea_admin}:{gitea_admin_password}@gitea:3000/api/v1/users/test/tokens \
+        -H 'Accept: application/json' -H 'Content-Type: application/json' \
+        -d {shlex.quote( '{"name":"token", "scopes":["all"]}' )} \
+        | jq -r '.sha1'
+    """).strip()
+
+    client.succeed(f"""
+      echo "http://{gitea_admin}:{gitea_admin_password}@gitea:3000" >~/.git-credentials-admin
+      git config --global credential.helper 'store --file ~/.git-credentials-admin'
+      git config --global user.email "test@client"
+      git config --global user.name "Test User"
+      git config --global gc.autodetach 0
+      git config --global gc.auto 0
+    """)
+  '';
+}