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.
diff --git a/src/main.c b/src/main.c
index 78b6122..e3db0b2 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1013,6 +1013,15 @@
 }
 
 /*
+ * 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
  * is closed.