oldtests: set nrformats to Vim default

src/nvim/testdir/setup.vim

@@ -11,6 +11,7 @@ set sidescroll=0
 set directory^=.
 set undodir^=.
 set backspace=
+set nrformats+=octal
 set nohidden smarttab noautoindent noautoread complete-=i noruler noshowcmd
 set listchars=eol:$
 set fillchars=vert:\|,fold:-

src/nvim/testdir/test_increment.vim

@@ -3,6 +3,7 @@
 func SetUp()
   new dummy
   set nrformats&vim
+  set nrformats+=octal
 endfunc
 
 func TearDown()