Add tests for ALLOCSPI.

1. Define XfrmUserSpiInfo struct
2. Add helper in xfrm.py to send XFRM_MSG_ALLOCSPI request
3. Write tests!
- allocate specific SPI
- allocate SPI from range
- allocate specific unavailable SPI
- allocate SPI from unavailable range

Bug: 62071755

Test: all_tests.sh passing on device kernels

Change-Id: I8ec5d68b3af230da4bebd5553fd865315f66d508