selftests: arg_parsing: Ensure data is flushed to disk before reading.

test_parse_test_list_file writes some data to
/tmp/bpf_arg_parsing_test.XXXXXX and parse_test_list_file() will read
the data back.  However, after writing data to that file, we forget to
call fsync() and it's causing testing failure in my laptop.  This patch
helps fix it by adding the missing fsync() call.

Fixes: 64276f01 ("selftests/bpf: Test_progs can read test lists from file")
Signed-off-by: Xing Guo <higuoxing@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251016035330.3217145-1-higuoxing@gmail.com