Make testing stubs fully thread-safe

This change makes the initialization of the hash upon missing key fully thread-safe. Before this change, initialization that would occur in two threads could overwrite each other, as illustrated here:

ruby-concurrency/concurrent-ruby#970

activesupport/lib/active_support/testing/time_helpers.rb

@@ -11,7 +11,9 @@ class SimpleStubs # :nodoc:
       Stub = Struct.new(:object, :method_name, :original_method)
 
       def initialize
-        @stubs = Concurrent::Map.new { |h, k| h[k] = {} }
+        @stubs = Concurrent::Map.new do |h, k|
+          h.compute_if_absent(k) { {} }
+        end
       end
 
       # Stubs object.method_name with the given block