Flag skipped Python command tests

[Lestropie]

Because of third party neuroimaging software dependencies, it is not always possible to execute all Python command tests successfully. Rather than treating these as "failures", it would be preferable to instead flag them as "skipped". One solution would be to use SKIP_REGULAR_EXPRESSION.

• Standardise error messages regarding absence of third-party software dependencies across MRtrix3 commands
  (May be best to use a centralised function)
• Add regular expression check to the function responsible for constructing Python command tests
• Ensure that if a CI Action is added to verify successful execution of all Python command tests prior to a tag update (CI: Conditional comprehensive testing of scripts #2134), the CI fails if there were any skipped tests (such an Action would need to embed all third-party dependencies).