add 128-EIA2 unit test using 33.401 Annex C.2 vectors

[Noahs212]

Noticed there's no unit test for srsran::security_128_eia2() even though
EIA1 and EIA3 both have one in lib/test/common/. EIA2 currently only
gets exercised through the PDCP integration tests, which can't really
catch an algorithm regression — encrypt+decrypt with the same broken
algorithm just cancels out.

Adds the three byte-aligned test vectors from 3GPP TS 33.401 v15.6.0
Annex C.2 (sets 2, 5, 8). Sets 5 and 8 happen to share inputs with
test_eia1.cc's test_set_4 / test_set_7, which is convenient —
it's the same crossover pattern the spec uses across EIA1/EIA2/EIA3.
Sets 1, 3, 4, 6 and 7 have non-byte-aligned message lengths and can't
go through the byte-length wrapper API.

Tested with make test_eia2 && ctest -R test_eia2 and confirmed the
test actually fails when an expected MAC byte is mutated.

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