[Tests] Update to new Makefile.plugin setup

- Declare explicitely the presence of tests/print.ml
- Compile it as dependency of `ptest_config` rather than
  `E_ACSL_TESTS` (in the latter case, they might not be compiled when
  `ptests` is launched, because `ptests` is launched by
  `E_ACSL_DEFAULT_TESTS` which is itself a dependency of `E_ACSL_TESTS`).