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/default.nix

@@ -0,0 +1,60 @@
+{ lib, config, ... }:
+{
+  name = "fetch-git";
+
+  imports = [
+    ./testsupport/gitea.nix
+  ];
+
+  /*
+  Test cases
+  The following is set up automatically for each test case:
+    - a repo with the {name} is created on the gitea server
+    - a repo with the {name} is created on the client
+    - the client repo is configured to push to the server repo
+  Python variables:
+    - repo.path: the path to the directory of the client repo
+    - repo.git: the git command with the client repo as the working directory
+    - repo.remote: the url to the server repo
+  */
+  testCases = [
+    {
+      name = "simple-http";
+      description = "can fetch a git repo via http";
+      script = ''
+        # add a file to the repo
+        client.succeed(f"""
+          echo chiang-mai > {repo.path}/thailand \
+          && {repo.git} add thailand \
+          && {repo.git} commit -m 'commit1'
+        """)
+
+        # memoize the revision
+        rev1 = client.succeed(f"""
+          {repo.git} rev-parse HEAD
+        """).strip()
+
+        # push to the server
+        client.succeed(f"""
+          {repo.git} push origin main
+        """)
+
+        # fetch the repo via nix
+        fetched1 = client.succeed(f"""
+          nix eval --impure --raw --expr "(builtins.fetchGit {repo.remote}).outPath"
+        """)
+
+        # check if the committed file is there
+        client.succeed(f"""
+          test -f {fetched1}/thailand
+        """)
+
+        # check if the revision is the same
+        rev1_fetched = client.succeed(f"""
+          nix eval --impure --raw --expr "(builtins.fetchGit {repo.remote}).rev"
+        """).strip()
+        assert rev1 == rev1_fetched
+      '';
+    }
+  ];
+}