selftests/bpf: Extract insert_test from parse_test_list

Split the logic to insert new tests into test filter sets out from
parse_test_list.

Fix the subtest insertion logic to reuse an existing top-level test
filter, which prevents the creation of duplicate top-level test filters
each with a single subtest.

Signed-off-by: Stephen Veiss <sveiss@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230427225333.3506052-2-sveiss@meta.com