The test tries to match -XX:+PrintFlagsFinal with the output of jhsdb -jmap --pid <pid> --heap. This fails when running ZGC, which leaves the MaxNewSize set to (size_t)-1.

$ ../build/fastdebug/jdk/bin/jhsdb jmap --pid 1550 --heap
Attaching to process ID 1550, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 15-internal+0-2020-01-20-0958066.stefank...

using thread-local object allocation.
ZGC with 20 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize = 32210157568 (30718.0MB)
   NewSize = 1363144 (1.2999954223632812MB)
   MaxNewSize = 17592186044415 MB
   OldSize = 5452592 (5.1999969482421875MB)
   NewRatio = 2
   SurvivorRatio = 8
   MetaspaceSize = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize = 17592186044415 MB
   G1HeapRegionSize = 0 (0.0MB)

and:
$ ../build/fastdebug/jdk/bin/java -XX:+PrintFlagsFinal -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -cp /localhome/tests/ HelloSleep 2>&1 | grep MaxNewSize
   size_t MaxNewSize = 18446744073709551615 {product} {default}

HeapSummary.printValMB looks like this:
   private static final double FACTOR = 1024*1024;
   private void printValMB(String title, long value) {
      if (value < 0) {
        System.out.println(alignment + title + (value >>> 20) + " MB");
      } else {
        double mb = value/FACTOR;
        System.out.println(alignment + title + value + " (" + mb + "MB)");
      }
   }

And when the value is negative we get a line that looks like this:
MaxNewSize = 17592186044415 MB

The test then tries to compare 17592186044415 with 18446744073709551615.

Repro:
make -C ../build/fastdebug test TEST=sun/tools/jhsdb/heapconfig/JMapHeapConfigTest.java JTREG="VM_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UseZGC"