CertificateTest: explicitly set validity check date

Since this test uses pre-generated certificates, they will expire
eventually given enough time. Instead of regenerating them, just set the
date in the PKIXParameters to compare their validity against.

Test: vogar --mode host --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-support-hostx_intermediates/classes.jack --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-hostdex_intermediates/classes.jack libcore/luni/src/test/java/tests/targets/security/cert/CertificateTest.java
Bug: 32756119
Bug: 32767302
Change-Id: Id48ecf289277ae9d98642b15aaa17c126bdc43e9