Unit test Vagrant::Util::Subprocess's STDIN support

- create new unit test file for this class, as none existed.
- test `Vagrant::Util::Subprocess#execute` behavior relating to STDIN
  handling.

test/unit/vagrant/util/subprocess_test.rb

@@ -0,0 +1,50 @@
+require File.expand_path("../../../base", __FILE__)
+require "vagrant/util/subprocess"
+
+describe Vagrant::Util::Subprocess do
+  describe '#execute' do
+    before do
+      # ensure we have `cat` and `echo` in our PATH so that we can run these
+      # tests successfully.
+      ['cat', 'echo'].each do |cmd|
+        if !Vagrant::Util::Which.which(cmd)
+          pending("cannot run subprocess tests without command #{cmd.inspect}")
+        end
+      end
+    end
+
+    let (:cat) { described_class.new('cat', :notify => [:stdin]) }
+
+    it 'yields the STDIN stream for the process if we set :notify => :stdin' do
+      echo = described_class.new('echo', 'hello world', :notify => [:stdin])
+      echo.execute do |type, data|
+        expect(type).to eq(:stdin)
+        expect(data).to be_a(::IO)
+      end
+    end
+
+    it 'can close STDIN' do
+      result = cat.execute do |type, stdin|
+        # We should be able to close STDIN without raising an exception
+        stdin.close
+      end
+
+      # we should exit successfully.
+      expect(result.exit_code).to eq(0)
+    end
+
+    it 'can write to STDIN correctly' do
+      data = "hello world\n"
+      result = cat.execute do |type, stdin|
+        stdin.write(data)
+        stdin.close
+      end
+
+      # we should exit successfully.
+      expect(result.exit_code).to eq(0)
+
+      # we should see our data as the output from `cat`
+      expect(result.stdout).to eq(data)
+    end
+  end
+end