RFR 8055262: Update jdk/test/java/util/Base64 tests to use java.util.Base64 instead of sun.misc.BASE64Encoder/Decoder

[Amy Lu]

There are two tests under jdk/test/java/util/Base64 that have dependency 
on internal API sun.misc.BASE64Encoder sun.misc.BASE64Decoder
java/util/Base64/TestBase64Golden.java
java/util/Base64/TestBase64.java

This fix is to remove the internal API dependency from the tests.

bug: https://bugs.openjdk.java.net/browse/JDK-8055262
webrev: http://cr.openjdk.java.net/~ewang/amylu/JDK-8055262/webrev.00/

As mentioned tests are testing java.util.Base64, in this case, the 
unnecessary sun.misc.BASE64* verifying can be removed.

Thanks,
Amy