Write unit tests and integration tests for the gemnasium-maven-plugin

Currently the https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-maven-plugin plugin does not have any tests.
Going forward we should write some tests to ensure this plugin will work correctly when we update it.

Implementation Plan:

• Add unit tests
• Add integration tests
• Add downstream pipeline to trigger the functional tests in https://gitlab.com/gitlab-org/security-products/tests/java-maven
• Add scheduled job to run test suite daily.
• Add java14 test job. https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-maven-plugin/-/blob/1eabc081be693607adbb3fa07d9c377be9aca359/.gitlab-ci.yml#L19-31

Testing

SET to add to the https://gitlab.com/gitlab-org/quality/secure-test-project-orchestrator/ pipelines