[CI] Add option --compare_to_input

• Related issues: None
• Requested reviewers: @kiene, @pihlajakuja

Reason why this change is needed

• It is relevant to compare to the input signal, e.g. for the PEAQ values.

Description of the change

• The --compare_to_input option is added to the tests. If used, the reference files are not used and do not need to be produced. The reference file is changed to the input before the call to cmp_pcm. You would run this twice, get two result tables for ref and dut, and compare those.
• With comparison to main, the input length will likely differ. The option also sets allow_differing_lengths. The behaviour is changed to pad to the longest file, rather than to truncate to the shortest. This should penalize the cases where a crash happened and a shorter file is compared to a longer file.

Affected operating points

• This option should not affect any of the existing tests.