test: Make 079-phantom less flakey.

Definitely block the main thread until the GC has cleared out the native
data objects.

Introduces new happens-before orderings which weren't there
previously:

* freeNativeStorage(dataA) happens-before printing "nulling 2"
* freeNativeStorage(dataB) happens-before bitmap shutdown

This removes the races that occurred between the printing of
"nulling 2" and "freeNativeStorage".

Bug: b/22287285
Test: art/test/run-test --host --optimizing 079-phantom
Change-Id: I04a4ecc2d3ac68470010b61d3eb1c2fedac55cca