Exclude more lines from test coverage (#1663)

These fall into a few categories:
- `_unreachable()`
- Verbose print messages
- Errors that should never practically occur (alloc/read/write failure,
  more than UINT32_MAX anonymous labels, etc)

src/link/assign.cpp

@@ -424,5 +424,5 @@ max_out:
 		}
 	}
 
-	unreachable_();
+	unreachable_(); // LCOV_EXCL_LINE
 }