reuse: license SEV-SNP test fixtures (CC0-1.0)

The binary/PEM SEV-SNP fixtures can't carry inline SPDX headers; annotate
them in REUSE.toml as CC0-1.0 alongside the existing nitro fixtures so the
REUSE compliance check passes.

Author: kvinwang

REUSE.toml

@@ -81,6 +81,9 @@ path = [
     "tpm-qvl/certs/gcp-root-ca.pem",
     "dstack-attest/tests/nitro_attestation.bin",
     "dstack-attest/tests/nitro_attestation_dbg.bin",
+    "dstack-attest/tests/sev_snp_attestation.bin",
+    "dstack-attest/tests/sev_snp_ask.pem",
+    "dstack-attest/tests/sev_snp_vcek.pem",
     "nsm-attest/tests/nitro_attestation.bin",
     "nsm-qvl/tests/nitro_attestation.bin",
     "nsm-qvl/certs/AWS_NitroEnclaves_Root-G1.pem",