ART: Separate out Realloc reuse and alignment tests

Separate out the tests for Realloc buffer reuse and realloc
alignment tests. This allows to turn off reuse tests under
memory tools (where we do not reuse).

Change-Id: I93375c6fbc577f9b796325f8143317610948455a
Test: m test-art-host-gtest-arena_allocator_test
Test: m valgrind-test-art-host-gtest-arena_allocator_test