Improve tests for thread dumps in JSON format

The tests for the new thread dump format at this time are:

test/hotspot/jtreg/serviceability/dcmd/thread/ThreadDumpToFileTest.java
test/jdk/com/sun/management/HotSpotDiagnosticMXBean/DumpThreads.java

These tests search the JSON text for strings but don't parse it completely. We can improve these tests with some test infrastructure that parses the thread dump. This test infrastructure is needed for the tests that will be added by JEP 428 too.