[PTESTS] simplify the heuristics of ptests

Since we use only `frama-c` which is native of byte when available:
- remove: if frama-c is called add the options
- remove: add PTESTS_FILE if it is not present

Adds variables to replace without hurdles:
- @FRAMA_C_PLUGINS_OPTIONS@: -autoload-plugins -plugins ... -load-module ...
- @OPTIONS@: the OPT
- @frama-c@: frama-c @FRAMA_C_PLUGINS_OPTIONS@ @PTEST_FILE@ @OPTIONS@