fetchGit: add simple test for ssh fetching

Also move tests to separate files which are auto-imported. This should allow people adding tests concurrently without introducing merge conflicts

tests/nixos/fetch-git/test-cases/ssh-simple/default.nix

@@ -0,0 +1,41 @@
+{
+  description = "can fetch a git repo via ssh";
+  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-ssh main
+    """)
+
+    # fetch the repo via nix
+    fetched1 = client.succeed(f"""
+      nix eval --impure --raw --expr '
+        (builtins.fetchGit "{repo.remote_ssh}").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_ssh}").rev
+      '
+    """).strip()
+    assert rev1 == rev1_fetched, f"rev1: {rev1} != rev1_fetched: {rev1_fetched}"
+  '';
+}