RFR(XS) 8218029 [TESTBUG] Use -Djava.class.path= to specify empty -cp in CDS tests

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8218029
http://cr.openjdk.java.net/~iklam/jdk13/8218029-testbug-empty-classpath.v01/

Please review this simple fix. For portability,

      "-cp", "\"\"",

in the JVM command-line is replaced with

      "-Djava.class.path="


Thanks!

Ioi