selftests/bpf: add pyperf scale test

Add a snippet of pyperf bpf program used to collect python stack traces
as a scale test for the verifier.

At 189 loop iterations llvm 9.0 starts ignoring '#pragma unroll'
and generates partially unrolled loop instead.
Hence use 50, 100, and 180 loop iterations to stress test.

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