test-unit depends on power_assert that the recent versions require
ruby 3.1 or later. While test-unit rescues syntax error at loading
power_assert for old ruby versions, warnings for experimental features
are not suppressed, and `assert_separately` fails because stderr is
not empty, by default. Since adding `required_ruby_version` to
power_assert causes the installation with old rubygems to fail, we
just ignore warnings for experimental features totally.
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