Testing of the admin-client modules in keycloak-client repository

[mposolda]

This task is about adding the tests to the keycloak-client repository for keycloak-admin-client .

The testsuite in keycloak-client should use junit5 approach instead of arquillian. The Keycloak server should be somehow provisioned and started during the testsuite. Some projects for the inspiration:

• https://github.com/keycloak/keycloak/tree/main/test-poc/
• https://github.com/keycloak-extensions/keycloak-openshift-ext/tree/main/src/test/java/org/keycloak/test/server (Ability to provision Keycloak server either during testsuite or have it already started on current laptop)

For the provisioning of Keycloak server, we may need support download specific version of Keycloak server. By default, it can be version 999.0.0-SNAPSHOT, which means that server will be provisioned from local maven repository or downloaded from latest nightly build if not available in the local maven repository.

In GH actions, we can probably test with the matrix of nightly and last 2 Keycloak released major versions, so that at least one Keycloak version corresponding to released RHBK server is tested.

The keycloak-admin-client should be backwards compatible with the older Keycloak releases (dedicated task for backwards compatibility #30818 ), so testing with last 2 Keycloak server releases should be OK.

Just to clarify: This task is NOT about updating the existing testsuite in the keycloak repository. The testsuite in the keycloak repository already contains big amount of tests for admin REST API, which all use java admin client. This testsuite will use the keycloak-admin-client-tests artifact, which will be inside the keycloak repository (Related task is #30796). That testsuite will remain unchanged.