vim-patch:8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes vim/vim#7269,
            closes vim/vim#6714)
59d8e56e04

Include minimal test_buffer.vim from patch 8.2.0243 for Test_balt().
Add entry for :balt to runtime/doc/index.txt from vim/vim#7819.

src/nvim/testdir/test_buffer.vim

@@ -0,0 +1,10 @@
+" Tests for Vim buffer
+
+func Test_balt()
+  new SomeNewBuffer
+  balt +3 OtherBuffer
+  e #
+  call assert_equal('OtherBuffer', bufname())
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab