pdl: Derive big-endian test files automatically

This allows us to update just a single file instead of keeping two
files in sync all the time.

Tag: #feature
Bug: 228306436
Test: atest pdl_rust_generator_tests_{le,be} pdl_python_generator_test
Change-Id: Ic8519e3bb07667513c73de07faa7e330836b641e