Test_SyncDefaultGems: Fail when editor run (ruby#9365)

When something went wrong and git launches editor, and a editor chosen
was terminal-based, a test run unnoticeably hangs.

As we intend editors not to be ran with --no-edit, GIT_EDITOR should be
defaulted to `false` so let Git command fails when it attempts to start a
editor. This allows catching such unintentional behaviour in test suite.

(Note: Prior to Git v2.32.0, git cherry-pick --no-edit doesn't work for
conflict resolution so currently the test hangs when older Git is in use.
git/git@39edfd5
i.e. Ubuntu 20.04, Debian Bullseye doesn't satisfy this prerequisite.)

tool/test/test_sync_default_gems.rb

@@ -175,13 +175,13 @@ def assert_sync(commits = true, success: true, editor: nil)
       result = nil
       out = capture_process_output_to([STDOUT, STDERR]) do
         Dir.chdir("src") do
-          if editor
-            editor, ENV["GIT_EDITOR"] = ENV["GIT_EDITOR"], editor
-            edit = true
-          end
+          orig_editor = ENV["GIT_EDITOR"]
+          ENV["GIT_EDITOR"] = editor || 'false'
+          edit = true if editor
+
           result = SyncDefaultGems.sync_default_gems_with_commits(@target, commits, edit: edit)
         ensure
-          ENV["GIT_EDITOR"] = editor if edit
+          ENV["GIT_EDITOR"] = orig_editor
         end
       end
       assert_equal(success, result, out)