patch 8.1.1759: no mode char for terminal mapping from maparg()

Problem:    No mode char for terminal mapping from maparg().
Solution:   Check for TERMINAL mode. (closes #4735)
diff --git a/src/testdir/test_maparg.vim b/src/testdir/test_maparg.vim
index 86e046b..f7524f5 100644
--- a/src/testdir/test_maparg.vim
+++ b/src/testdir/test_maparg.vim
@@ -29,6 +29,12 @@
         \ 'nowait': 1, 'expr': 0, 'sid': sid, 'lnum': lnum + 1, 'rhs': 'bar',
 	\ 'buffer': 1},
         \ maparg('foo', '', 0, 1))
+  let lnum = expand('<sflnum>')
+  tmap baz foo
+  call assert_equal({'silent': 0, 'noremap': 0, 'lhs': 'baz', 'mode': 't',
+        \ 'nowait': 0, 'expr': 0, 'sid': sid, 'lnum': lnum + 1, 'rhs': 'foo',
+	\ 'buffer': 0},
+        \ maparg('baz', 't', 0, 1))
 
   map abc x<char-114>x
   call assert_equal("xrx", maparg('abc'))