[Cleanup] Redundant substring() operation in tips.test

Removes the second substring() parameter if this parameter is the length
of the string. It's the default value.

Found during testing of Bug 569800 (new JDT cleanup for this case).

Change-Id: I19679d43411eaa3c306e690b762d177a020c37a7
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>