docs: Add tests/README.md

Author: roxblnfk

tests/README.md

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+# Testing Testo
+
+Testo is a modular framework.
+Each module will eventually become a separate Composer package. Until that happens,
+the test folder structure should be maintained in a way that allows for easy migration
+of tests to separate module repositories in the future.
+
+For example, the `tests/Assert` folder contains tests for the Assert module.
+The `tests/Assert/suites.php` file lists the test suite configurations for this module.
+Currently, these configurations are merged into the main testing config `testo.php`,
+integrating the Assert module tests into the overall Testo testing process.
+
+Tests must be isolated from other tests, meaning each module should have
+its own fixtures, mock objects, etc.
+
+## Self Tests
+
+In addition to the commonly known types of tests (unit tests, integration tests, etc.),
+Testo has another type of test - Self Tests.
+
+Example of a test that tests itself:
+
+```php
+#[Test]
+public function numbers(): void
+{
+    Assert::equals(1, 1);
+    Assert::equals(1, 1.0);
+    Assert::equals(1.0, 1);
+    Assert::equals("2", 2);
+}
+```
+
+If the test completes with Passed status, we have verified that:
+- `Assert::equals` registers a successful assertion in the Test State, otherwise the test would be marked as Risky.
+- `Assert::equals` doesn't fail on the provided values.
+
+Special attributes will soon be added for Self Tests to simplify their creation,
+for example `#[Testing\ExpectStatus(Status::Failed)]` and others.