vim-patch:8.2.3510: changes are only detected with one second accuracy

Problem:    Changes are only detected with one second accuracy.
Solution:   Use the nanosecond time if possible.  (Leah Neukirchen,
            closes vim/vim#8873, closes vim/vim#8875)
0a7984af56

In Nvim Test_checktime_fast() is also flaky. Add a delay to avoid that.

src/nvim/eval/funcs.c

@@ -4539,6 +4539,7 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     "mouse",
     "multi_byte",
     "multi_lang",
+    "nanotime",
     "num64",
     "packages",
     "path_extra",