Update CDS tests in anticipation of JDK-8270489

For testing JDK-8270489 (Support archived heap objects in EpsilonGC), some CDS tests should be updated:

[1] Add "@require vm.gc == null" to mark test cases that will fail if a collector is explicitly specified using -XX:+UseXXXGC

[2] Rename this WhiteBox API to reflect what it checks:

    !WB.areSharedStringsIgnored() -> WB.areSharedStringsMapped()

With EpsilonGC, the shared strings are not "ignored", but they are also not "mapped" either (they are copied into the Java heap). The existing tests are for mapped regions only, so they should check for WB.areSharedStringsMapped().

[3] For test cases that check if a heap object is a shared interned string, instead of the generic WB.isShared(Object o) API, use a new specific API WB.isSharedInternedString(String s).

[4] Also fix the two test cases that were sensitive to irregular -Xlog output:

test/hotspot/jtreg/runtime/cds/ServiceLoaderTest.java: remove timestamp decorations from the logs

test/hotspot/jtreg/runtime/cds/appcds/DumpClassList.java: handle extra space characters that may appear in the tag decoration between square brackets.