tests: better handling of tests without a sav file

In tests, do not tag tests without sav files with *skipped*.
Instead: an empty `.res` file is a *success*, otherwise a non-empty `.res` is a *failure*.

In order to enable the correct behavior for all tests, two special `.skip` files are added to catch regressions in the Nit code.

`cc.skip` skips the C compilation and is used for programs that require a specific but unavailable C library.
Tests in this file are compiled with the `--no-cc` option.

`exec.skip` skips the execution and is used for program whose execution is unsuitable or unreproducible.
Programs that rely on the environ var `NIT_TESTING` in their code may want to drop it and just add themselves to the `exec.skip`