feat: support icons in testname format

[kdkasad]

Adds support for --format-icons when using --format testname.

Notable points:

• The default icons are now used if none are specified, rather than text like before. This matches the documentation.
• Two spaces are used between the icon and the package name, just like in the pkgname format. This applies even when the text icons are used, as it seems unwise to add more fragile edge cases.
• Empty packages use the skip icon instead of the pass icon, matching previous behavior.
  • For the text icons only, EMPTY is used instead of SKIP in this case, again matching previous behavior.

Unit tests have been updated/added as appropriate to guarantee the above points are accurate.

The github-actions format is unchanged; it required some refactoring to separate from the testname format but the behavior stays the same.

Closes #399.