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.

When a process with multiple threads is forked, LSAN outputs warning
messages to stderr like:

    ==276855==Running thread 276851 was not suspended. False leaks are possible.

We should ignore messages like this in tests.