Fix nonce uniqueness tests

std::unique consolidates the unique nonces at the start of the `nonces`
vector, but it doesn't modify nonces.size(), so these tests weren't
actually verifying that the nonces were unique. Add a vector::erase
call to shrink the vector.

After upgrading libc++, std::unique is [[nodiscard]] and this bug is
a compiler error.

Bug: 175635923
Test: treehugger
Test: m VtsAidlSharedSecretTargetTest VtsHalKeymasterV4_0TargetTest
Change-Id: I7fd8c40a3920bf3a8988c8065503c78ba36dc742