refactor(test): rename alter_slashes, invert its behavior

- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
  pervasive convention, not `\` slashes.

test/functional/terminal/buffer_spec.lua

@@ -342,7 +342,7 @@ describe(':terminal buffer', function()
     command('wincmd p')
 
     -- cwd will be inserted in a file URI, which cannot contain backs
-    local cwd = fn.getcwd():gsub('\\', '/')
+    local cwd = t.fix_slashes(fn.getcwd())
     local parent = cwd:match('^(.+/)')
     local expected = '\027]7;file://host' .. parent
     api.nvim_chan_send(term, string.format('%s\027\\', expected))