activate! & deactivate! state properly in tests (which allows easy replication of bug reported in #24)

test/fakefs_test.rb

@@ -4,9 +4,14 @@ class FakeFSTest < Test::Unit::TestCase
   include FakeFS
 
   def setup
+    FakeFS.activate!
     FileSystem.clear
   end
 
+  def teardown
+    FakeFS.deactivate!
+  end
+
   def test_can_be_initialized_empty
     fs = FileSystem
     assert_equal 0, fs.files.size

test/safe_test.rb

@@ -5,6 +5,10 @@ class FakeFSSafeTest < Test::Unit::TestCase
     FakeFS.deactivate!
   end
 
+  def teardown
+    FakeFS.activate!
+  end
+
   def test_FakeFS_method_does_not_intrude_on_global_namespace
     path = '/path/to/file.txt'