Disable thread safe switch for fast and rubocop specs

What does this MR do and why?

Both spec helpers fast_spec_helper and rubocop_spec_helper load spec/support/rspec.

Follow-up of !102748 (merged).

How to set up and validate locally

Apply path

diff --git a/spec/rubocop/cop_todo_spec.rb b/spec/rubocop/cop_todo_spec.rb
index c641001789fc..89741f96e265 100644
--- a/spec/rubocop/cop_todo_spec.rb
+++ b/spec/rubocop/cop_todo_spec.rb
@@ -4,6 +4,10 @@
 require_relative '../../rubocop/cop_todo'
 
 RSpec.describe RuboCop::CopTodo do
+  it 'works' do
+    RSpec.configure { p _1.threadsafe }
+  end
+
   let(:cop_name) { 'Cop/Rule' }
 
   subject(:cop_todo) { described_class.new(cop_name) }

and run

$ bin/rspec spec/rubocop/cop_todo_spec.rb  -e works
Run options: include {:full_description=>/works/}

Randomized with seed 42704

RuboCop::CopTodo
  # order random
false
  works

Finished in 0.00101 seconds (files took 1.36 seconds to load)
1 example, 0 failures

Randomized with seed 42704

MR acceptance checklist

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