Trim too long assert messages to prevent line overflow

[prantlf]

Attempts to fix #30.

Print this:

✓ Exported dependency names can be resolved.
✓ Resolving an unknown dependency fails.
# pass  3
✓ ok

instead of this:

✓ Exported dependency names can be resolved.
# Resolving an unknown dependency fails.
✓ Resolving an unknown dependency fails.ncy: "dummy"." should start with "Unre# tests 3
# pass  3
✓ ok

Fixes #30.