Avoid tainting the default config

[theMarix]

When running tests, depending on the order they are discovered, the default_config can become tainted before the test_default_config test is run. This causes this test to fail.

Admittedly its likely never an issue when using ntfy as a script. But its in general a good practice not to direcly use mutable objects as default values and less flaky tests are easier to handle.