patch 8.0.0846: cannot get the name of the pty of a job

Problem:    Cannot get the name of the pty of a job.
Solution:   Add the "tty" entry to the job info. (Ozaki Kiichi, closes #1920)
            Add the term_gettty() function.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 48ee1c4..cc5afc3 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -30,6 +30,12 @@
 
 func Test_terminal_basic()
   let buf = Run_shell_in_terminal()
+  if has("unix")
+    call assert_match("^/dev/", job_info(g:job).tty)
+    call assert_match("^/dev/", term_gettty(''))
+  else
+    call assert_equal("", job_info(g:job).tty)
+  endif
   call Stop_shell_in_terminal(buf)
   call term_wait(buf)