Summary:
When using parameterized tests, `gtest` sometimes prints the test
parameters. If no other printing method is available, it essentially
produces a hex dump of the object. This can cause issues with valgrind
with types like `TestArgs` in `table_test`, where the object layout has
gaps (with uninitialized contents) due to the members' alignment
requirements. The patch fixes the uninitialized reads by providing an
`operator<<` for `TestArgs` and also makes sure all members are
initialized (in a consistent order) on all code paths.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6980

Test Plan: `valgrind --leak-check=full ./table_test`

Reviewed By: siying

Differential Revision: D22045536

Pulled By: ltamasi

fbshipit-source-id: 6f5920ac28c712d0aa88162fffb80172ed769c32