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/setup.nix

@@ -0,0 +1,106 @@
+{ lib, config, extendModules, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+
+  indent = lib.replaceStrings ["\n"] ["\n    "];
+
+  execTestCase = testCase: ''
+
+    ### TEST ${testCase.name}: ${testCase.description} ###
+
+    with subtest("${testCase.description}"):
+        repo = Repo("${testCase.name}")
+        ${indent testCase.script}
+  '';
+in
+{
+
+  options = {
+    setupScript = mkOption {
+      type = types.lines;
+      description = ''
+        Python code that runs before the main test.
+
+        Variables defined by this code will be available in the test.
+      '';
+      default = "";
+    };
+    testCases = mkOption {
+      description = ''
+        The test cases. See `testScript`.
+      '';
+      type = types.listOf (types.submodule {
+        options.name = mkOption {
+          type = types.str;
+          description = ''
+            The name of the test case.
+
+            A repository with that name will be set up on the gitea server and locally.
+
+            This name can also be used to execute only a single test case via:
+              `nix build .#hydraJobs.fetch-git.{test-case-name}`
+          '';
+        };
+        options.description = mkOption {
+          type = types.str;
+          description = ''
+            A description of the test case.
+          '';
+        };
+        options.script = mkOption {
+          type = types.lines;
+          description = ''
+            Python code that runs the test.
+
+            Variables defined by `setupScript` will be available here.
+          '';
+        };
+      });
+    };
+  };
+
+  config = {
+    nodes.client = {
+      environment.variables = {
+        _NIX_FORCE_HTTP = "1";
+      };
+      nix.settings.experimental-features = ["nix-command" "flakes"];
+    };
+    setupScript = ''
+      class Repo:
+        """
+        A class to create a git repository on the gitea server and locally.
+        """
+        def __init__(self, name):
+          self.name = name
+          self.path = "/tmp/repos/" + name
+          self.remote = "http://gitea:3000/test/" + name
+          self.git = f"git -C {self.path}"
+          self.create()
+
+        def create(self):
+          gitea.succeed(f"""
+            curl --fail -X POST http://{gitea_admin}:{gitea_admin_password}@gitea:3000/api/v1/user/repos \
+              -H 'Accept: application/json' -H 'Content-Type: application/json' \
+              -d {shlex.quote( f'{{"name":"{self.name}", "default_branch": "main"}}' )}
+          """)
+          client.succeed(f"""
+            mkdir -p {self.path} \
+            && git init -b main {self.path} \
+            && {self.git} remote add origin {self.remote}
+          """)
+    '';
+    testScript = ''
+      start_all();
+
+      ${config.setupScript}
+
+      ### SETUP COMPLETE ###
+
+      ${lib.concatStringsSep "\n" (map execTestCase config.testCases)}
+    '';
+  };
+}