OsOperations::set_env is corrected

[dmitry-lipetsk]

Declaration:

    def set_env(
        self,
        var_name: str,
        var_val: typing.Optional[str],
    ) -> None:

LocalOperations:

• deletes variable when value is None via 'os.environ.pop(var_name, None)'

RemoteOperations:

• [add] T_ENVS
• [add] _remote_env
• [add] _remote_env_guard
• exec_command processes self._remote_env
• set_env saves new value into self._remote_env

Two new tests:

• test_set_env__persistence
• test_set_env__evil