selftests/bpf: Add tests for kprobe attachment with duplicate symbols

bpf_fentry_shadow_test exists in both vmlinux (net/bpf/test_run.c) and
bpf_testmod (bpf_testmod.c), creating a duplicate symbol condition when
bpf_testmod is loaded. Add subtests that verify kprobe behavior with
this duplicate symbol:

In attach_probe:
- dup-sym-{default,legacy,perf,link}: unqualified attach succeeds
  across all four modes, preferring vmlinux over module shadow.
- MOD:SYM qualification attaches to the module version.

In kprobe_multi_test:
- dup_sym: kprobe_multi attach with kprobe and kretprobe succeeds.

bpf_fentry_shadow_test is not invoked via test_run, so tests verify
attach and detach succeed without triggering the probe.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Link: https://lore.kernel.org/r/20260407203912.1787502-3-andrey.grodzovsky@crowdstrike.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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