mojo leveldb: Reenable tests which can be moved to the in-memory db.
mojo leveldb: Reenable tests which can be moved to the in-memory db.

File operations were causing flake on some of the leveldb
tests. We now run tests that can be done on an in-memory database there.

(There's still one test left disabled which requires file operations and
will be sorted out separately.)

BUG=602820

Review-Url: https://codereview.chromium.org/1928073002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390747}

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