Unit tests are as important to a firmware/program (“application”) as the production source code. As with the regular code, you need to give careful thought to where the tests reside, both physically and logically, in relation to the code under test. If you put unit tests in the wrong place, the tests you have written may not be run.
Similarly, if you do not devise ways to reuse parts of your tests and use test hierarchy, you will end up with test code that is either unmaintainable or hard to understand. So, what makes a good test?
Tests should be independent and repeatable. It is a pain to debug a test that succeeds or fails as a result of other tests. PlatformIO isolates the tests by running each of them as a separate application. When a test fails, PlatformIO allows you to run it in isolation for quick debugging.
Tests should not depend on the main application source code.
In PlatformIO, each test is an independent application and should contain its
own main() function (setup() / loop() for Arduino,
app_main() for Espressif IoT Development Framework).
Linking the main application source code from src_dir
with a test suite code will lead to multiple compilation errors.
Hence, the Shared Code is disabled by default.
Tests should be well organized and reflect the structure of the tested code.
PlatformIO lets you organize tests using nested folders. The only folder with
a name prefixed by test_ is nominated for unit testing and is an independent
test/application. See Test Hierarchy.