tools/auto_system_test: Add comments.

tools/auto_system_test

@@ -1,13 +1,24 @@
 #!/usr/bin/env expect
 
+# Run the confidence test non-interactively.  Since the script
+# asks for passphrases, we use "expect" to simulate keyboard data entry.
+
+# Run the test:
 spawn tools/confidence_test.sh
 
+# As we run the confidence test, respond with the right password.
+# We do this for up to 300 times to prevent an infinite loop.
+
 set times 0;
 while { $times < 300 } {
    expect {
+       # The script outputs what the password will be, and we save
+       # that info in $pw any time we see the text.
        "my password is the lowercase letter a" { set pw "a\n" ; exp_continue }
        "my password is the lowercase letter b" { set pw "b\n" ; exp_continue }
+       # If the passphrase is requested, send it.
        "Passphrase:" { send $pw ; exp_continue }
+       # If we reach EOF, exit this loop.
        eof { break }
    }
    set times [ expr $times+1];