Replace SecureRandom.random_bytes by Random.bytes in cache store tests

It cause cause /dev/urandom exhaustion on some systems (mostly macOS).

activesupport/test/cache/behaviors/cache_store_behavior.rb

@@ -254,7 +254,7 @@ def test_incompressible_data
 
     # generate an incompressible string
     loop do
-      incompressible = SecureRandom.random_bytes(1.kilobyte)
+      incompressible = Random.bytes(1.kilobyte)
       break if incompressible.bytesize < Zlib::Deflate.deflate(incompressible).bytesize
     end