Add additional virtual-memory test cases for tags: ensure that anonymous
memory types maintain tags for a casual store/reload to mapped pages with
PROT_READ | PROT_WRITE.  We don't push the VM system to the point of
paging, just casual faulting, so there's much more to test here.

There is one XFAIL case in which we would like MAP_SHARED mappings of
files to not accept stores of tagged capabilities, but the VM system does
not currently enforce this as it does not (currently) manage the
capability-disable bits in the TLB.