Refactor tests to be isolated.

test/helper.vim

@@ -0,0 +1,15 @@
+set runtimepath+=../
+source ../plugin/gitgutter.vim
+
+function! Setup()
+  call system('git checkout fixture.txt')
+  edit! fixture.txt
+  sign unplace *
+endfunction
+
+function! DumpSigns(filename)
+  execute 'redir! > ' a:filename.'.out'
+    silent execute 'sign place'
+  redir END
+endfunction
+