Use a temporary directory for the VAGRANT_CWD when running tests

Tests before were picking up a Vagrantfile in the Vagrant source
directory, which can cause some funny failures. This ensures that each
test run will actually establish a new temporary CWD so that a
Vagrantfile is hopefully never found.

test/unit/base.rb

@@ -9,6 +9,7 @@ require "vagrant"
 $:.unshift File.expand_path("../../", __FILE__)
 
 # Load in helpers
+require "support/tempdir"
 require "unit/support/shared/base_context"
 
 # Do not buffer output
@@ -19,3 +20,7 @@ $stderr.sync = true
 RSpec.configure do |c|
   c.expect_with :rspec, :stdlib
 end
+
+# Configure VAGRANT_CWD so that the tests never find an actual
+# Vagrantfile anywhere, or at least this minimizes those chances.
+ENV["VAGRANT_CWD"] = Tempdir.new.path

test/unit/support/shared/base_context.rb

@@ -27,4 +27,23 @@ shared_context "unit" do
 
     return Pathname.new(f.path)
   end
+
+  # This helper provides temporary environmental variable changes.
+  def with_temp_env(environment)
+    # Build up the new environment, preserving the old values so we
+    # can replace them back in later.
+    old_env = {}
+    environment.each do |key, value|
+      old_env[key] = ENV[key]
+      ENV[key]     = value
+    end
+
+    # Call the block, returning its return value
+    return yield
+  ensure
+    # Reset the environment no matter what
+    old_env.each do |key, value|
+      ENV[key] = value
+    end
+  end
 end

test/unit/vagrant/environment_test.rb

@@ -13,7 +13,9 @@ describe Vagrant::Environment do
 
   describe "current working directory" do
     it "is the cwd by default" do
-      described_class.new.cwd.should == Pathname.new(Dir.pwd)
+      with_temp_env("VAGRANT_CWD" => nil) do
+        described_class.new.cwd.should == Pathname.new(Dir.pwd)
+      end
     end
 
     it "is set to the cwd given" do
@@ -23,7 +25,12 @@ describe Vagrant::Environment do
     end
 
     it "is set to the environmental variable VAGRANT_CWD" do
-      pending "A good temporary ENV thing"
+      directory = File.dirname(__FILE__)
+      instance = with_temp_env("VAGRANT_CWD" => directory) do
+        described_class.new
+      end
+
+      instance.cwd.should == Pathname.new(directory)
     end
 
     it "raises an exception if the CWD doesn't exist" do