env_test.cc defines EnvPosixTest which tests the Env implementation returned by Env::Default(). The naming is a bit unfortunate, as the tests in env_test.cc are written against the Env contract, and therefore are applicable to any Env implementation. An instance of the confusion caused by the naming is [] which added a dependency from env_test.cc to EnvPosixTestHelper, which is closely coupled to EnvPosix.

This change disentangles EnvPosix-specific test code into a env_posix_test.cc file. The code there uses EnvPosixTestHelper and specifically targets the EnvPosix implementation. env_test.cc now implements EnvTest, and contains tests that are also applicable to other ports, which may define their own Env implementation.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148914642