test.apex is copied from ADBD apex built in AOSP.

$ apksigner verify -v test.apex
Verifies
Verified using v1 scheme (JAR signing): false
Verified using v2 scheme (APK Signature Scheme v2): false
Verified using v3 scheme (APK Signature Scheme v3): true
Verified using v4 scheme (APK Signature Scheme v4): false
Verified for SourceStamp: false
Number of signers: 1

APK files are copied from tools/apksig/src/test/resources/com/android/apksig/.