ruby : fix test failures in test_whisper (#2955)

ruby : fix test failures in test_whisper (#2955)

* bindings.ruby : fix test failures in test_whisper

This commit updates the parallel tests to use 2 processors instead of
the number of processors on the system. It also comments out the setting
of the log callback to an empty lambda as this causes a segfault when
enabled.

The motivation for the change to the number of processors is that if one
has a large number of processors, for example I have 16 on the machine I
used to test this, this would cause the following warning to be printed:
```console
whisper_full_with_state: input is too short - 680 ms < 1000 ms. consider padding the input audio with silence
```

This is logged from:
```c++
int whisper_full_with_state(
        struct whisper_context * ctx,
          struct whisper_state * state,
    struct whisper_full_params   params,
                   const float * samples,
                           int   n_samples) {
   ...
    if (seek_end < seek_start + 100) {
        WHISPER_LOG_WARN("%s: input is too short - %d ms < 1000 ms. consider padding the input audio with silence\n", __func__, (seek_end - seek_start)*10);
        return 0;
    }
```
This will return early and there will be segment callbacks to be invoked
which in turn will cause the tests to fail.

* bindings.ruby : fix warnings in tests

This commit fixes the following warnings in the Ruby tests:
```console
/whisper/bindings/ruby/tests/test_segment.rb:52:
warning: ambiguity between regexp and two divisions:
wrap regexp in parentheses or add a space after `/' operator
```
And also adds a '_' prefix to some unused variables to avoid warnings.

* bindings.ruby : enable Wisper.log_set in tests

The commit reverts the commenting out of the Whisper.log_set call in
the test_whisper.rb tests.

I'm no longer getting segfaults when running the tests with this
which was the case earlier. One theory could be that I rebased this to
include the latest ggml sync to master to make sure things still worked.
With the latest changes in ggml, I can't reproduce the segfaults.