(FACT-3133) allow for echo on Windows

`echo` was supported for Windows in facter 3, but has been missing
from support in facter 4; this change allows for custom facts to
use the builtin echo commands in cmd.exe or powershell.

Author: tvpartytonight

lib/facter/custom_facts/core/execution/windows.rb

@@ -9,6 +9,10 @@ def search_paths
         DEFAULT_COMMAND_EXTENSIONS = %w[.COM .EXE .BAT .CMD].freeze
 
         def which(bin)
+          # `echo` is allowed for facter 3.x compatibility, otherwise
+          # all commands much be found on the PATH or absolute.
+          return bin if /^echo$/i =~ bin
+
           if absolute_path?(bin)
             return bin if File.executable?(bin)
           else

spec/custom_facts/core/execution/windows_spec.rb

@@ -36,6 +36,16 @@
       allow(ENV).to receive(:[]).with('PATHEXT').and_return nil
     end
 
+    context 'when trying to use builtin windows commands' do
+      it 'allows echo' do
+        expect(executor.which('echo')).to eq 'echo'
+      end
+
+      it 'disallows other builtin windows commands' do
+        expect(executor.which('dir')).to eq nil
+      end
+    end
+
     context 'when it is provided with an absolute path' do
       it 'returns the path to binary if executable' do
         allow(File).to receive(:executable?).with('C:\Tools\foo.exe').and_return true