patch 8.2.4943: changing 'switchbuf' may have no effect

Problem:    Changing 'switchbuf' may have no effect.
Solution:   Handle 'switchbuf' in didset_string_options(). (Sean Dewar,
            closes #10406)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 8854ffd..0a53315 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -1299,4 +1299,15 @@
   set wildoptions=
 endfunc
 
+func Test_switchbuf_reset()
+  set switchbuf=useopen
+  sblast
+  call assert_equal(1, winnr('$'))
+  set all&
+  call assert_equal('', &switchbuf)
+  sblast
+  call assert_equal(2, winnr('$'))
+  only!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab