docs: Add information about how to significantly improve the test suite (#1472)

* docs: Add information about how to significantly improve the test suite

Hi,

this iPR adds an inromation for the page about `JWT` authentication about how to make the tests fast.

The issue boils down to the very slow password hashers that are used in tests by default.

Each time we start a new project, or review the current one, we have to do it, because tests are extremely slow.

This is only one of the many optimizations available for the end developer, but let's start here. Other ones are listed in by blog post: https://maks-rafalko.github.io/blog/2021-11-21/symfony-tests-performance/#using-more-simple-password-hasher

* Update wording in core/jwt.md

Co-authored-by: Alan Poulain <[email protected]>

Co-authored-by: Alan Poulain <[email protected]>

Author: maks-rafalko

core/jwt.md

@@ -344,3 +344,26 @@ class AuthenticationTest extends ApiTestCase
 ```
 
 Refer to [Testing the API](../distribution/testing.md) for more information about testing API Platform.
+
+### Improving Tests Suite Speed
+
+Since now we have a `JWT` authentication, functional tests require us to log in each time we want to test an API endpoint. This is where [Password Hashers](https://symfony.com/doc/current/security/passwords.html) come into play.
+
+Hashers are used for 2 reasons:
+
+1. To generate a hash for a raw password (`self::$container->get('security.user_password_hasher')->hashPassword($user, '$3CR3T')`)
+2. To verify a password during authentication
+
+While hashing and verifying 1 password is quite a fast operation, doing it hundreds or even thousands of times in a tests suite becomes a bottleneck, because reliable hashing algorithms are slow by their nature.
+
+To significantly improve the test suite speed, we can use more simple password hasher specifically for the `test` environment.
+
+```yaml
+# override in api/config/packages/test/security.yaml for test env
+security:
+    password_hashers:
+        App\Entity\User:
+            algorithm: md5
+            encode_as_base64: false
+            iterations: 0
+```