Relax RSS limit in test_named_structs_are_not_rooted

The default limit of 2.0 is too tight for RSS measurements on macOS,
causing random CI failures when the ratio lands just above 2.0 (e.g.
2.05). Increase to 2.2 to account for normal RSS measurement noise.

https://github.com/ruby/ruby/actions/runs/23923072484/job/69773771596

    1) Failure:
  TestStruct::TopStruct#test_named_structs_are_not_rooted [/Users/runner/work/ruby/ruby/src/test/ruby/test_struct.rb:541]:
  rss: 8781824 => 18006016.
  Expected 2.050373134328358 to be < 2.0.

  ruby -v: ruby 4.1.0dev (2026-04-02T21:34:07Z master e957b3a) +PRISM [arm64-darwin23]

Author: k0kubun

test/ruby/test_struct.rb

@@ -538,7 +538,7 @@ def test_named_structs_are_not_rooted
     omit 'skip on riscv64-linux CI machine. See https://github.com/ruby/ruby/pull/13422' if ENV['RUBY_DEBUG'] == 'ci' && /riscv64-linux/ =~ RUBY_DESCRIPTION
 
     # [Bug #20311]
-    assert_no_memory_leak([], <<~PREP, <<~CODE, rss: true)
+    assert_no_memory_leak([], <<~PREP, <<~CODE, rss: true, limit: 2.2)
       code = proc do
         Struct.new("A")
         Struct.send(:remove_const, :A)