patch 8.0.0539: startup test fails on Mac

Problem:    Startup test fails on Mac.
Solution:   Use another term name, "unknown" is known. Avoid a 2 second delay.

src/main.c

@@ -1012,6 +1012,15 @@ common_init(mparm_T *paramp)
 #endif
 }
 
+/*
+ * Return TRUE when the --not-a-term argument was found.
+ */
+    int
+is_not_a_term()
+{
+    return params.not_a_term;
+}
+
 /*
  * Main loop: Execute Normal mode commands until exiting Vim.
  * Also used to handle commands in the command-line window, until the window