diff --git a/src/testdir/setup.vim b/src/testdir/setup.vim
new file mode 100644
index 0000000..f7e475a
--- /dev/null
+++ b/src/testdir/setup.vim
@@ -0,0 +1,11 @@
+" Common preparations for running tests.
+
+" Make sure 'runtimepath' does not include $HOME.
+set rtp=$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after
+
+" Only when the +eval feature is present. 
+if 1
+  " Make sure $HOME does not get read or written.
+  let $HOME = '/does/not/exist'
+endif
+