vim-patch:8.2.0426: some errors were not tested for

Problem:    Some errors were not tested for.
Solution:   Add tests. (Dominique Pelle, closes vim/vim#5824)
9b9be007e7

Cherry-pick get_highest_fnum() from patch 8.1.1908 to make tests pass.

src/nvim/buffer.c

@@ -94,11 +94,19 @@ static char *e_buflocked = N_("E937: Attempt to delete a buffer that is in use")
 // Number of times free_buffer() was called.
 static int buf_free_count = 0;
 
+static int top_file_num = 1;            ///< highest file number
+
 typedef enum {
   kBffClearWinInfo = 1,
   kBffInitChangedtick = 2,
 } BufFreeFlags;
 
+/// @return  the highest possible buffer number
+int get_highest_fnum(void)
+{
+  return top_file_num - 1;
+}
+
 /// Read data from buffer for retrying.
 ///
 /// @param read_stdin  read file from stdin, otherwise fifo
@@ -1644,8 +1652,6 @@ void no_write_message_nobang(const buf_T *const buf)
 // functions for dealing with the buffer list
 //
 
-static int top_file_num = 1;            ///< highest file number
-
 /// Initialize b:changedtick and changedtick_val attribute
 ///
 /// @param[out]  buf  Buffer to initialize for.