chore(testing): add example for testcontainers

Related to CAM-11281

TESTING.md

@@ -130,6 +130,7 @@ Docker image can be used this way, please perform the following steps:
    * If you use a private Docker repository, please include it in the Docker image name (e.g. private.registry.org/postgres)
 1. In the `pom.xml` file located in the `./database` folder, check out the `database.tc.url` property to ensure that 
    the Docker tags match.
+1. Make sure that the `testcontainers` profile is added to your Maven `settings.xml` (you can find it [here](settings/maven/nexus-settings.xml)).
 
 At the moment, Testcontainers can be used with the Camunda-supported versions of the following databases. Please make 
 sure that the database image is configured according to [this guide](https://docs.camunda.org/manual/latest/user-guide/process-engine/database/database-configuration/#isolation-level-configuration):
@@ -139,6 +140,12 @@ sure that the database image is configured according to [this guide](https://doc
 * CockroachDB
 * MS-SQL 2017/2019 ([MSSQL-specific configuraion guide](https://docs.camunda.org/manual/latest/user-guide/process-engine/database/mssql-configuration/))
 
+To execute the process engine test suite with a certain database (e.g. PostgreSQL), you should call Maven in the 
+engine directory with
+```shell
+mvn clean test -Ppostgresql,testcontainers
+```
+
 # Limiting the Number of Engine Unit Tests
 
 Due to the fact that the number of unit tests in the camunda engine increases daily and that you might just want to test a certain subset of tests the maven-surefire-plugin is configured in a way that you can include/exclude certain packages in your tests.