integration-tests: Reduce the `TRANSPORT_TEST_BUFFER_WRITABLE_LENGTH`

The `TRANSPORT_TEST_BUFFER_WRITABLE_LENGTH` is set to `2048`. Reduce it
to `1350`, so that the test data can be sent in a single ethernet
packet.

Without this change, it takes a very long time to complete transport
tests and most of the times test fails.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>