Change tests to use separate configurations for `cargo test`

[StoneDot]

Currently, the cargo test command uses the default configuration directory, $HOME/.dynein. This means that our configuration is replaced after cargo test. This is not good from the perspective of the developer's experience. We want to fix this behavior by using individual configurations for tests.

[ryota-sakamoto]

We can use DYNEIN_CONFIG_DIR to load configuration on other directory instead of default directory.

https://github.com/awslabs/dynein/blob/748c68ec683e7e7798ec9b1f2130a040589d15dc/src/app.rs#L649

The env is already used on the specific test so we can expand it to all of the tests.

https://github.com/awslabs/dynein/blob/748c68ec683e7e7798ec9b1f2130a040589d15dc/tests/auxiliary.rs#L47