Fix traverse_files_upwards to make searching up to the root possible

Previously, the meaning of `FileFinder.root_level` would be "first non
searched directory", now it is "last searched directory".

This is more intuitive in my opinion (I assumed this behavior from the
beginning until I found out it was not like that), but also more
"correct", because it allows you to search up to the root directory if
`FileFinder.root_level` is set to "/".

Note that this `FileFinder.root_level?` logic is only useful for testing to
provide an isolated environment not affected by the external file system
where the tests are running. When rubocop runs "normally", it's ignored.

However, in follow up work, I plan to start using these code paths, and
this change allows me to stop artificially passing the parent of the
directory where I want to stop searching.

lib/rubocop/file_finder.rb

@@ -32,10 +32,10 @@ def find_last_file_upwards(filename, start_dir)
 
     def traverse_files_upwards(filename, start_dir)
       Pathname.new(start_dir).expand_path.ascend do |dir|
-        break if FileFinder.root_level?(dir)
-
         file = dir + filename
         yield(file.to_s) if file.exist?
+
+        break if FileFinder.root_level?(dir)
       end
     end
   end

lib/rubocop/rspec/shared_contexts.rb

@@ -12,16 +12,17 @@
       # get mismatched pathnames when loading config files later on.
       tmpdir = File.realpath(tmpdir)
 
+      virtual_home = File.expand_path(File.join(tmpdir, 'home'))
+      Dir.mkdir(virtual_home)
+      ENV['HOME'] = virtual_home
+      ENV.delete('XDG_CONFIG_HOME')
+
+      working_dir = File.join(tmpdir, 'work')
+
       # Make upwards search for .rubocop.yml files stop at this directory.
-      RuboCop::FileFinder.root_level = tmpdir
+      RuboCop::FileFinder.root_level = working_dir
 
       begin
-        virtual_home = File.expand_path(File.join(tmpdir, 'home'))
-        Dir.mkdir(virtual_home)
-        ENV['HOME'] = virtual_home
-        ENV.delete('XDG_CONFIG_HOME')
-
-        working_dir = File.join(tmpdir, 'work')
         Dir.mkdir(working_dir)
 
         Dir.chdir(working_dir) do