Size the IMEX test fixtures' flush matrix per nozzle

The IMEX fixtures set seven nozzles but kept multifilament_config's single
filaments x filaments flush block. get_flush_volumes_matrix splits that block
across the nozzles, leaving each with 7 values, and
ToolOrdering::reorder_extruders_for_minimum_flush_volume then reads them as a
7 x 7 matrix, past the end of the buffer. One of the affected tests segfaulted
on Windows x64; ASan reproduces the overflow in that test on Linux, where it
passed only by luck.

The fixtures now repeat the block once per nozzle, as the GUI does, and size
flush_multiplier to match, since append_full_config takes the nozzle count from
it. The helper is shared in test_helpers.

Co-authored-by: HanifKoh <76276251+HanifKoh@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fff_print/test_imex_mode_gcode.cpp

@@ -114,6 +114,7 @@ static void imex_7x4_printer(DynamicPrintConfig &config)
         // compares the two scripts and nothing else.
         { "gcode_flavor",              "klipper" },
     });
+    size_flush_to_nozzles(config);
 }
 
 // Set the mode scripts positionally, one per name in imex_mode_names. Assigning the vector