fix(terminal): check size when switching buffers

Problem: terminal not always resized when switching to its buffer.
Solution: add missing calls to terminal_check_size.

Adjust screen test for v0.11.
(cherry picked from commit e56292071a)

src/nvim/buffer.c

@@ -1772,6 +1772,10 @@ void enter_buffer(buf_T *buf)
   }
   curbuf->b_last_used = time(NULL);
 
+  if (curbuf->terminal != NULL) {
+    terminal_check_size(curbuf->terminal);
+  }
+
   redraw_later(curwin, UPD_NOT_VALID);
 }