audio/libxmp: Update to 4.7.3 and enable 'make test'

• Add USES= pathfix
• Convert PORTVERSION to DISTVERSION
• Add "make test"
• Disable static lib
• Remove unnecessary strip command

Changelog: https://github.com/libxmp/libxmp/releases/tag/libxmp-4.7.3

PR: 298424
Reviewed by: ehaupt (maintainer)