testing/mepo: don't depend on a specific font

mepo-scripts uses icons from nerd fonts in its user interface. For this to work properly, it needs to depend on a font that provides these symbols.

Currently, mepo-scripts depends on font-hack-nerd. When font-hack-nerd is installed, it will affect which font fontconfig chooses as a fallback in other applications.

Change the dependency to font-nerd-fonts-symbols, which ships only the nerd font symbols. This covers the need in mepo-scripts without affecting any other use case.