core: randomize perm test filename

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 BUG FIXES:
 
+  - core: Randomize some filenames internally to improve the parallelism
+      of Vagrant. [GH-3386]
   - core: Don't error if network problems cause box update check to
       fail [GH-3391]
   - guests/fedora: Fix hostname setting. [GH-3382]

lib/vagrant/environment.rb

@@ -516,7 +516,8 @@ module Vagrant
 
       # Attempt to write into the home directory to verify we can
       begin
-        path = @home_path.join("perm_test")
+        suffix = (0...32).map { (65 + rand(26)).chr }.join
+        path   = @home_path.join("perm_test_#{suffix}")
         path.open("w") do |f|
           f.write("hello")
         end