Use Dir.home to avoid platform dependency

On Solaris 10, `echo ~#{ENV['USER']}` As expected

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66176 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/file/expand_path_spec.rb

@@ -89,7 +89,7 @@ describe "File.expand_path" do
   platform_is_not :windows do
     before do
       @var_home = ENV['HOME'].chomp('/')
-      @db_home = `/bin/echo ~#{ENV['USER']}`.chomp
+      @db_home = Dir.home
     end
 
     # FIXME: these are insane!