From Working Effectively With Legacy Code by Feathers, p. 14[0]:
Unit tests run fast. If they don’t run fast, they aren’t unit tests.
Other kinds of tests often masquerade as unit tests. A test is not a unit test if:
1. It talks to a database.
2. It communicates across a network.
3. It touches the file system.
4. You have to do special things to your environment (such as editing configuration files) to run it.
Tests that do these things aren’t bad. Often they are worth writing, and you generally will write them in unit test harnesses. However, it is important to be able to separate them from true unit tests so that you can keep a set of tests that you can run fast whenever you make changes.
Unit tests run fast. If they don’t run fast, they aren’t unit tests.
Other kinds of tests often masquerade as unit tests. A test is not a unit test if:
1. It talks to a database.
2. It communicates across a network.
3. It touches the file system.
4. You have to do special things to your environment (such as editing configuration files) to run it.
Tests that do these things aren’t bad. Often they are worth writing, and you generally will write them in unit test harnesses. However, it is important to be able to separate them from true unit tests so that you can keep a set of tests that you can run fast whenever you make changes.
[0]: https://www.google.com/books/edition/Working_Effectively_wit...