[tests] rewrite test sed filter in Python for better portability

nix/kernel-tests.nix

tests/syntax/cpp-command.c

tests/syntax/cpp-command.t/cpp-command.c

tests/syntax/cpp-command.t/filter.py

tests/syntax/cpp-command.t/run.t

tests/syntax/dune

tests/syntax/oracle/cpp-command.0.res.oracle

tests/syntax/oracle/cpp-command.1.res.oracle

tests/syntax/oracle/cpp-command.2.res.oracle

tests/syntax/oracle/cpp-command.3.res.oracle

tests/syntax/oracle/cpp-command.4.res.oracle

tests/syntax/oracle/cpp-command.5.res.oracle

tests/syntax/oracle/cpp-command.6.res.oracle