patch 8.0.0716: not easy to start Vim cleanly

Problem:    Not easy to start Vim cleanly without changing the viminfo file.
            Not possible to know whether the -i command line flag was used.
Solution:   Add the --clean command line argument.  Add the 'viminfofile'
            option.  Add "-u DEFAULTS".

src/testdir/test_channel_write.py

@@ -0,0 +1,18 @@
+#!/usr/bin/python
+#
+# Program that writes a number to stdout repeatedly
+#
+# This requires Python 2.6 or later.
+
+from __future__ import print_function
+import sys
+import time
+
+if __name__ == "__main__":
+
+    done = 0
+    while done < 10:
+        done = done + 1
+        print(done)
+        sys.stdout.flush()
+        time.sleep(0.05)  # sleep 50 msec