Revive the disabled fff_print test suite (#14196)

* Fix null-deref and arranger bugs that gate headless slicing tests

export_gcode dereferenced a null result out-param, enum serialization
dereferenced a null keys_map, and get_arrange_polys left bed_idx unseeded so
the arranger dropped items. All only affect the headless test/CLI path.

* Fix the headless test harness and add G-code test helpers

Use the real arranger, fix temp-file handling with an RAII guard, and add
layers_with_role / max_z for inspecting sliced G-code.

* Re-enable the Model construction test

* Re-enable SupportMaterial tests and add an enforced-support test

* Re-enable and extend PrintObject layer-height and perimeter tests

* Re-enable Print skirt, brim, and solid-surface tests

* Re-enable and extend PrintGCode tests

Un-hide the basic scenario (dead-key fixes, reframes, trimmed trivia) and add
initial-layer-height, sequential-order, and null-result export tests.

* Re-enable and reframe the skirt/brim tests

Detect skirt/brim by G-code role comment instead of a sentinel speed, and
resolve the previously-unfinished skirt-enclosure test.

* Replace the stale lift()/unlift() test with a z_hop test

* Delete the stub and broken Flow tests

src/libslic3r/GCode.cpp

@@ -2194,7 +2194,7 @@ void GCode::do_export(Print* print, const char* path, GCodeProcessorResult* resu
     BOOST_LOG_TRIVIAL(info) << "Exporting G-code finished" << log_memory_info();
     print->set_done(psGCodeExport);
     
-    if(is_BBL_Printer())
+    if(is_BBL_Printer() && result != nullptr)
         result->label_object_enabled = m_enable_exclude_object;
     // Write the profiler measurements to file
     PROFILE_UPDATE();