[tests] dune: fix test directory 'syntax'

tests/syntax/Refresh_visitor.i

tests/syntax/aggressive_merging_1.i

tests/syntax/copy_logic.i

tests/syntax/copy_visitor.i

tests/syntax/cpp-command.c

tests/syntax/extern_init.i

tests/syntax/formals_decl_leak.i

tests/syntax/local-init-const.i

tests/syntax/loop-case-switch-for-unroll.c

tests/syntax/multiple_decls_contracts.c

tests/syntax/string_concat.c

tests/syntax/unroll_labels.i

tests/syntax/wstring_concat.c