RFR 8200241: [TESTBUG] java/lang/String/concat/ tests should not force source/release = 9 anymore

[Aleksey Shipilev]

Bug:
 https://bugs.openjdk.java.net/browse/JDK-8200241

Webrev:
  http://cr.openjdk.java.net/~shade/8200241/webrev.01/

As discussed in a separate thread on this list, we do not need source/target = 
9 in these tests anymore.

Testing: make run-test TEST=test/jdk/java/lang/String/concat/, Linux x86_64 
{fastdebug|release}

Thanks,
-Aleksey