Fix #269 Set $TMPDIR to a default value, if not set

test/test_tmpdir_init.vader

@@ -0,0 +1,2 @@
+Execute($TMPDIR should be set to a default value if unset):
+  AssertEqual '/tmp', $TMPDIR

test/vimrc

@@ -20,3 +20,7 @@ set foldnestmax=10
 set ttimeoutlen=0
 
 let g:mapleader=','
+
+" Clear the TMPDIR value for tests.
+" The plugin should set this to /tmp by default, which we will test.
+let $TMPDIR = ''