RFR(S): 8185531: [TESTBUG] Improve test configuration for shared strings

[Mikhailo Seledtsov]

Please review this simple improvement to add 2 more relevant test 
configurations to shared strings tests: -XX:+UseStringDeduplication  and 
-XX:-CompactStrings

     JBS: https://bugs.openjdk.java.net/browse/JDK-8185531
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8185531.00/
     Testing:
         1. Locally: ran the affected tests locally (Linux-x64)
            All PASS
         2. Automated multi-platform test system:
            Same set: test/hotspot/jtreg/runtime/appcds/sharedStrings/
            All PASS

Thank you,
Misha