[ruby/test-unit-ruby-core] Fix for rubies older than 2.7

https://github.com/ruby/test-unit-ruby-core/commit/c53dddaa9c

tool/lib/core_assertions.rb

@@ -365,7 +365,7 @@ eom
         args.insert((Hash === args.first ? 1 : 0), "-w", "--disable=gems", *$:.map {|l| "-I#{l}"})
         args << "--debug" if RUBY_ENGINE == 'jruby' # warning: tracing (e.g. set_trace_func) will not capture all events without --debug flag
         # power_assert 3 requires ruby 3.1 or later
-        args << "-W:no-experimental" if RUBY_VERSION < "3.1."
+        args << "-W:no-experimental" if ("2.7."..."3.1.").cover?(RUBY_VERSION)
         stdout, stderr, status = EnvUtil.invoke_ruby(args, src, capture_stdout, true, **opt)
 
         if sanitizers&.lsan_enabled?