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/lua/uri_spec.lua

@@ -217,7 +217,7 @@ describe('URI methods', function()
         ]],
         file
       )
-      local expected_uri = 'file:///' .. file:gsub('\\', '/')
+      local expected_uri = 'file:///' .. t.fix_slashes(file)
       eq(expected_uri, exec_lua(test_case))
       os.remove(file)
     end)