[eacsl:tests] Allow the use of `PTESTS_OPTS` to change the test configuration

- The makefile updates `PTESTS_OPTS` instead of `PLUGIN_PTESTS_OPTS` so
  that modifications of `PTESTS_OPTS` by the user are taken into
  account.
- All the test config files are generated at the same time.
- The test dependencies of E-ACSL are added to the target
  `plugins_ptests_config` to be generated along with the main target.
  This in turn allow the user to directly use `ptests.opt` to launch
  tests after compiling the plugin.