Factorize processing tests

PreCourlis/processing/interpolate_lines.py

scripts/prepare_profiles.py

...data/expected/import_tracks_first_axis_point_abs_long.gml

test/data/expected/interpolate_lines.gml

test/data/expected/interpolate_lines_with_constraints.gml

test/data/expected/interpolate_lines_with_constraints.xsd

test/processing/__init__.py

test/processing/test_export_courlis.py

test/processing/test_export_mascaret.py

test/processing/test_import_georef.py

test/processing/test_import_tracks.py

test/processing/test_interpolate_lines.py

test/processing/test_interpolate_points.py

test/processing/test_lines_to_points.py

test/processing/test_points_to_lines.py

test/processing/test_prepare_tracks.py