test/old: skip Test_strptime() on Windows

POSIX strptime does not exist in Windows.
There is a C++ workaround but I don't know how to use it.
Julia ported BSD's "strptime()" but I can't compile the file
or embed the relevant code into src/nvim/os/time.c

I cannot use "#ifdef" in eval.lua because of function hashing.
"#ifdef" is required to point "strptime()" to NULL
such that "CheckFunction strptime" fails.

src/nvim/testdir/test_functions.vim

@@ -217,6 +217,7 @@ endfunc
 
 func Test_strptime()
   CheckFunction strptime
+  CheckNotMSWindows
 
   if exists('$TZ')
     let tz = $TZ