tests: net: ip-addr: Fix compiler warning

Building with Zephyr SDK 0.16.5 revealed a minor bug with the buffer
size provided to snprintk():
  specified bound 38 exceeds destination size 20 [-Wstringop-overflow=]

As we provide the buffer to snprintk() with an offset, the actual
buffer size should be reduced by that offset value.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>