Docs about db:truncate and TestUtils.db().truncate() (#196)

* docs: about db:truncate and TestUtils.db().truncate()

* Apply suggestions from code review

Co-authored-by: Romain Lanz <2793951+RomainLanz@users.noreply.github.com>

Co-authored-by: Romain Lanz <2793951+RomainLanz@users.noreply.github.com>

content/guides/testing/introduction.md

@@ -203,6 +203,8 @@ Make sure you have `@adonisjs/lucid` installed for the following examples to wor
 
 ### Migrating database
 
+#### Reset database after each run cycle
+
 You can migrate the database before running all the tests and roll it back after the tests. This can be done by registering the `TestUtils.db().migrate()` hook within the `tests/bootstrap.ts` file.
 
 ```ts
@@ -218,6 +220,32 @@ export const runnerHooks: Required<Pick<Config, 'setup' | 'teardown'>> = {
 }
 ```
 
+#### Truncate database after each run cycle
+
+An alternative to the above approach is to truncate all tables in the database after each run cycle instead of rolling it back. This can be done by registering the `TestUtils.db().truncate()` hook within the `tests/bootstrap.ts` file.
+
+```ts
+// title: tests/bootstrap.ts
+export const runnerHooks: Required<Pick<Config, 'setup' | 'teardown'>> = {
+  setup: [
+    () => TestUtils.ace().loadCommands(),
+    // highlight-start
+    () => TestUtils.db().truncate()
+    // highlight-end
+  ],
+  teardown: [],
+}
+```
+
+Before running your tests, the hook will migrate the database if necessary. After the tests are run, all the tables in your database will be kept but truncated. 
+
+So next time you run your tests, your database will be empty but will not need to be migrated again. This may be a better approach and will save you some time if you have a lot of migrations.
+
+:::tip
+Note that the hook internally calls the `node ace db:truncate` command that you can also run manually. Also, note that this command will truncate all tables **except** `adonis_schema` and `adonis_schema_versions` tables.
+:::
+
+
 ### Seeding database
 You can also run database seeders by calling the `TestUtils.db().seed()` method.