ApkVerityTestApp is a test helper app to be installed with fs-verity signature file (.fsv_sig). In order for this CTS test to run on a release build across vendors, the signature needs to be verified against a release certificate loaded to kernel.
Modifying the test helper app will also require to sign the apk with a local debug key. You will also need to point the test to use your local build.
On debuggable build, it can be done by:
adb root adb shell 'mini-keyctl padd asymmetric fsv-play .fs-verity' < fsverity-debug.x509.der
On user build, the keyring is closed and doesn't accept extra key. A workaround is to copy the .der file to /system/etc/security/fsverity. Upon reboot, the certificate will be loaded to kernel as usual.
TODO: provide instruction once the test switches to prebuilt and release signature.