selftests/bpf: convert bpf_verif_scale.c to sub-tests API

Expose each BPF verifier scale test as individual sub-test to allow
independent results output and test selection.

Test run results now look like this:

  $ sudo ./test_progs -t verif/
  #3/1 loop3.o:OK
  #3/2 test_verif_scale1.o:OK
  #3/3 test_verif_scale2.o:OK
  #3/4 test_verif_scale3.o:OK
  #3/5 pyperf50.o:OK
  #3/6 pyperf100.o:OK
  #3/7 pyperf180.o:OK
  #3/8 pyperf600.o:OK
  #3/9 pyperf600_nounroll.o:OK
  #3/10 loop1.o:OK
  #3/11 loop2.o:OK
  #3/12 strobemeta.o:OK
  #3/13 strobemeta_nounroll1.o:OK
  #3/14 strobemeta_nounroll2.o:OK
  #3/15 test_sysctl_loop1.o:OK
  #3/16 test_sysctl_loop2.o:OK
  #3/17 test_xdp_loop.o:OK
  #3/18 test_seg6_loop.o:OK
  #3 bpf_verif_scale:OK

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>