selftests/bpf: Move is_jit_enabled() into testing_helpers

Currently, is_jit_enabled() is only used in test_progs, move it into
testing_helpers so that it can be used in test_verifier. While at it,
remove the second argument "0" of open() as Hou Tao suggested.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Hou Tao <houtao1@huawei.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20240123090351.2207-2-yangtiezhu@loongson.cn