v5: trigger transmit failure in the test_buf_allocator

In test_buf_allocator, if the transmit buffer length is less than
ETH_HLEN, returns transmit failure. This will enable testing of the
interpreter's error handling logic within the unit test.

Test: TH
Change-Id: Ib05297c986dcb732a99e2f25c3e8acbd14d00ebc
2 files changed