Increase test coverage for pmd-apex

This is project has a good test coverage. However, we can add more tests to increase the test coverage to 60.0%.

➜  pmd-apex git:(master) go test -cover ./...
PASS | gitlab.com/gitlab-org/security-products/analyzers/pmd-apex/v2 0.180s
COVER| 54.8% [#####_____]
     | ok       gitlab.com/gitlab-org/security-products/analyzers/pmd-apex/v2/metadata  0.130s  coverage: [no statements]
PASS | gitlab.com/gitlab-org/security-products/analyzers/pmd-apex/v2/plugin 0.294s
COVER| 84.6% [########__]