selftests/bpf: Temporarily disable part of btf_dump:var_data test.

Commit 7443b296 ("x86/percpu: Move cpu_number next to current_task")
moved global per_cpu variable 'cpu_number' into pcpu_hot structure.
Therefore this part of var_data test is no longer valid.
Disable it until better solution is found.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>