Revert "assert_separately: Suppress experimental warnings"

This reverts commit c2dd855.
test-unit 3.7.2 squashes the warnings.

Author: nobu

lib/core_assertions.rb

@@ -339,8 +339,6 @@ def assert_separately(args, file = nil, line = nil, src, ignore_stderr: nil, **o
         args = args.dup
         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."
         stdout, stderr, status = EnvUtil.invoke_ruby(args, src, capture_stdout, true, **opt)
 
         if sanitizers&.lsan_enabled?

test-unit-ruby-core.gemspec

@@ -23,5 +23,5 @@ Gem::Specification.new do |spec|
   ]
   spec.require_paths = ["lib"]
 
-  spec.add_runtime_dependency("test-unit")
+  spec.add_runtime_dependency("test-unit", ">= 3.7.2")
 end