CipherTest: add tests for RSA OAEP with labels

The OAEP spec allows you to specify a label L for indicating what the
key should be used for, etc. This is represented here as a
PSource.PSpecified instance. Add a couple tests for these to make sure
they're functioning correctly.

Test: vogar --mode host --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-support-hostdex_intermediates/classes.jack --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-hostdex_intermediates/classes.jack libcore/luni/src/test/java/libcore/javax/crypto/CipherTest.java
Change-Id: I440c55cdbbe6c59b4a41d538cc4b81f186450917