test_migrator: fix lock file leak

Closes Homebrew/homebrew#43025.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/test/test_migrator.rb

@@ -93,6 +93,8 @@ class MigratorTests < Homebrew::TestCase
     rmtree HOMEBREW_PREFIX/"opt" if (HOMEBREW_PREFIX/"opt").directory?
     # What to do with pin?
     @new_f.unpin
+
+    FormulaLock::LOCKDIR.children.each(&:unlink)
   end
 
   def test_move_cellar