Summary:
Allow compaction_job_test, db_io_failure_test, dbformat_test, deletefile_test, and fault_injection_test to use a custom Env object. Also move ```RegisterCustomObjects``` declaration to a header file to simplify things.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9087

Test Plan: Run manually using "buck test rocksdb/src:compaction_job_test_fbcode" etc.

Reviewed By: riversand963

Differential Revision: D32007222

Pulled By: anand1976

fbshipit-source-id: 99af58559e25bf61563dfa95dc46e31fa7375792