Restore capture_stderr helper used by JSON formatter specs

The helper was removed in 2729bdf when it had no callers. The
concurrent-overwrite warning specs added in f9b698a depend on it,
so the merged tip of main fails on every Ruby version with
NoMethodError: undefined method 'capture_stderr'.

Author: sferik

spec/helper.rb

@@ -16,3 +16,12 @@ def source_fixture(filename)
 def source_fixture_base_directory
   @source_fixture_base_directory ||= File.dirname(__FILE__)
 end
+
+def capture_stderr
+  previous_stderr = $stderr
+  $stderr = StringIO.new
+  yield
+  $stderr.string
+ensure
+  $stderr = previous_stderr
+end