unittests: avoid using pattern matching on file names

The directory name could contain special characters that trips up the
matching used by find.  Instead, let's just make sure that the filename
starts with the directory name.

test/unit/tempfile_spec.lua

@@ -62,7 +62,7 @@ describe('tempfile related functions', function()
     itp('generate file name in Neovim own temp directory', function()
       local dir = vim_gettempdir()
       local file = vim_tempname()
-      assert.truthy(file:find('^' .. dir .. '[^/]*$'))
+      eq(string.sub(file, 1, string.len(dir)), dir)
     end)
   end)
 end)