test: suppress PCR public key auto-loading in TEST-70-TPM2 dditest

test: suppress PCR public key auto-loading in TEST-70-TPM2 dditest

The dditest block calls systemd-repart with Encrypt=tpm2 but without
--tpm2-public-key-pcrs=. Since systemd-stub drops
/run/systemd/tpm2-pcr-public-key.pem when booting from a signed UKI
systemd-repart auto-loads it and enrolls a signed PCR policy, and
then systemd-cryptsetup tpm2-device=auto has no matching signature file,
so unlock fails.

--tpm2-public-key= is not enough as the default kicks in then.

Follow-up for cd18656d47710c251a44a8f5f9d616151a909152