tests: fix username conflicts in parallel tests.

Parallel tests sometimes failed because the username generator sometimes
generated duplicate usernames.

test/factories/post.rb

@@ -1,8 +1,6 @@
 FactoryBot.define do
   factory(:post) do
-    sequence :md5 do |n|
-      n.to_s
-    end
+    md5 { SecureRandom.hex(32) }
     uploader
     uploader_ip_addr {"127.0.0.1"}
     tag_string {"tag1 tag2"}

test/factories/user.rb

@@ -1,8 +1,6 @@
 FactoryBot.define do
   factory(:user, aliases: [:creator, :updater]) do
-    sequence :name do |n|
-      "user#{n}"
-    end
+    name { FFaker::Internet.user_name + $PID.to_s }
     password {"password"}
     default_image_size {"large"}
     level {20}