libgit2: Enable tests

The tests hanging comment dates from 2018.
I did encounter a slow test when running them in a nix-shell -
the nonetwork::bad_urls one.
The name suggests that it might work fine, but it's still annoying
when building in nix-shell, and it seems unlikely that this
particular test will catch anything for us.

pkgs/development/libraries/libgit2/default.nix

@@ -46,7 +46,22 @@ stdenv.mkDerivation rec {
 
   propagatedBuildInputs = lib.optional (!stdenv.isLinux) libiconv;
 
-  doCheck = false; # hangs. or very expensive?
+  doCheck = true;
+  checkPhase = ''
+    testArgs=(-v -xonline)
+
+    # slow
+    testArgs+=(-xclone::nonetwork::bad_urls)
+
+    # failed to set permissions on ...: Operation not permitted
+    testArgs+=(-xrepo::init::extended_1)
+    testArgs+=(-xrepo::template::extended_with_template_and_shared_mode)
+
+    (
+      set -x
+      ./libgit2_tests ''${testArgs[@]}
+    )
+  '';
 
   passthru.tests = {
     inherit libgit2-glib;