Fix page_heap_test flakiness

After 71fa9f87 (use 2-level page map for 48-bit addresses), there is a
potential for allocs of additional leaves in the middle of the test.
With ASLR enabled on amd64 (without TCMALLOC_SMALL_BUT_SLOW), this
results in the test failing sometimes (< 1% of the time). Instead, make
sure all those leaves are allocated before deciding on the baseline heap
usage in the test.