Default to using leveldb as storage engine in tests.

Switching to leveldb caused a few tests to start failing, because
memstore has a different model of conflicts that leveldb.  Those tests
were fixed so they pass with both leveldb and memstore.

You can still run the tests with memstore, by running:

    export STORAGE_ENGINE=memstore
    make test

Change-Id: I6e080f2df6942d3239d8d383c861e263f778f9cc
2 files changed