patch 8.1.1365: source command doesn't check for the sandbox

Problem:    Source command doesn't check for the sandbox. (Armin Razmjou)
Solution:   Check for the sandbox when sourcing a file.
diff --git a/src/testdir/test_source.vim b/src/testdir/test_source.vim
index a33d286..5166baf 100644
--- a/src/testdir/test_source.vim
+++ b/src/testdir/test_source.vim
@@ -36,3 +36,12 @@
   au! SourcePre
   au! SourcePost
 endfunc
+
+func Test_source_sandbox()
+  new
+  call writefile(["Ohello\<Esc>"], 'Xsourcehello')
+  source! Xsourcehello | echo
+  call assert_equal('hello', getline(1))
+  call assert_fails('sandbox source! Xsourcehello', 'E48:')
+  bwipe!
+endfunc