patch 8.2.2979: not all options code is covered by tests

Problem:    Not all options code is covered by tests.
Solution:   Add more tests for options. (Yegappan Lakshmanan, closes #8369)
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index 60753a6..de82b5b 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -593,6 +593,12 @@
   if has('unix')
     call assert_fails('cd `pwd`', 'E48:')
   endif
+  " some options cannot be changed in a sandbox
+  call assert_fails('set exrc', 'E48:')
+  call assert_fails('set cdpath', 'E48:')
+  if has('xim')
+    call assert_fails('set imstyle', 'E48:')
+  endif
 endfunc
 
 func Test_sandbox()