DEV: Use rspec mocks to properly verify a race condition

This is a small followup of https://github.com/discourse/discourse/pull/28124.

spec/rails_helper.rb

@@ -186,7 +186,18 @@ end
 PER_SPEC_TIMEOUT_SECONDS = 45
 BROWSER_READ_TIMEOUT = 30
 
+# To avoid erasing `any_instance` from Mocha
+require "rspec/mocks/syntax"
+RSpec::Mocks::Syntax.singleton_class.define_method(:enable_should) { |*| nil }
+RSpec::Mocks::Syntax.singleton_class.define_method(:disable_should) { |*| nil }
+
+RSpec::Mocks::ArgumentMatchers.remove_method(:hash_including) # We’re currently relying on the version from Webmock
+
 RSpec.configure do |config|
+  config.expect_with :rspec do |c|
+    c.syntax = :expect
+  end
+
   config.fail_fast = ENV["RSPEC_FAIL_FAST"] == "1"
   config.silence_filter_announcements = ENV["RSPEC_SILENCE_FILTER_ANNOUNCEMENTS"] == "1"
   config.extend RedisSnapshotHelper
@@ -206,10 +217,18 @@ RSpec.configure do |config|
   config.include ServiceMatchers
   config.include I18nHelpers
 
-  config.mock_framework = :mocha
   config.order = "random"
   config.infer_spec_type_from_file_location!
 
+  config.mock_with :rspec do |mocks|
+    mocks.verify_partial_doubles = true
+    mocks.verify_doubled_constant_names = true
+    mocks.syntax = :expect
+  end
+  config.mock_with MultiMock::Adapter.for(:mocha, :rspec)
+
+  config.include Mocha::API
+
   if ENV["GITHUB_ACTIONS"]
     # Enable color output in GitHub Actions
     # This eventually will be `config.color_mode = :on` in RSpec 4?