chore(ext): document exceptions to generic extension tests

This helps us identify why certain extensions do not use the generic tests.

Author: jfroche

nix/ext/tests/http.nix

@@ -158,3 +158,6 @@ self.inputs.nixpkgs.lib.nixos.runTest {
       check_upgrade_path("17")
     '';
 }
+# We don't use the generic test for this extension because:
+# http is not using semver versioning scheme, so we need to adapt the version checks
+# otherwise the test fails with ERROR:  extension "http" has no installation script nor update path for version "1.5.0"

nix/ext/tests/pg_repack.nix

@@ -164,3 +164,6 @@ self.inputs.nixpkgs.lib.nixos.runTest {
         test.check_upgrade_path("17")
     '';
 }
+# We don't use the generic test for this extension because:
+# pg_repack does not support upgrade as the extension doesn't provide the upgrade SQL scripts
+# and fails with ERROR:  extension "pg_repack" has no update path from version "1.4.8" to version "1.5.0"

nix/ext/tests/pgrouting.nix

@@ -230,3 +230,4 @@ self.inputs.nixpkgs.lib.nixos.runTest {
       check_upgrade_path("orioledb-17")
     '';
 }
+# We don't use the generic test for this extension because: it requires postgis to be installed as well.