selftests/bpf: Split module_attach into subtests

The test verifies attachment to various hooks in a kernel module,
however, everything is flattened into a single test. This makes it
impossible to run or skip test cases selectively.

Isolate each BPF program into a separate subtest. This is done by
disabling auto-loading of programs and loading and testing each program
separately.

At the same time, modernize the test to use ASSERT* instead of CHECK and
replace `return` by `goto cleanup` where necessary.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/20260225120904.1529112-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>