Squish: Always use the same function for preparing templates

tests/system/shared/project.py

tests/system/shared/suites_qtta.py

tests/system/suite_general/tst_create_proj_wizard/test.py

tests/system/suite_qtquick/tst_qml_editor/test.py

tests/system/suite_qtquick/tst_qtquick_creation2/test.py