sync LVI tests
The LVI tests for the `x86_64-fortanix-unknown-sgx` target have gotten out of sync. LVI is still mitigated correctly, but the LVI tests need minor modifications. Other (non LVI-related) tests fail when the target applies LVI mitigations as they assume the generated code contains forbidden instructions such as `retq`. These tests are ignored for the sgx environment.
cc: `@jethrogb`