[11] 2020-07-17, Bulk backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200717/webrev.01/

This backports simple fixes, mostly improvements in tests:

 [backport] 8248041: Shenandoah: pre-Full GC root updates may miss some roots
 [backport] 8247860: Shenandoah: add update watermark line in rich assert failure message
 [backport] 8247751: Shenandoah: options tests should run with smaller heaps
 [backport] 8247754: Shenandoah: mxbeans tests can be shorter
 [backport] 8247757: Shenandoah: split heavy tests by heuristics to improve parallelism

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey