selftests/bpf: add sub-tests support for test_progs

Allow tests to have their own set of sub-tests. Also add ability to do
test/subtest selection using `-t <test-name>/<subtest-name>` and `-n
<test-nums-set>/<subtest-nums-set>`, as an extension of existing -t/-n
selector options. For the <test-num-set> format: it's a comma-separated
list of either individual test numbers (1-based), or range of test
numbers. E.g., all of the following are valid sets of test numbers:
  - 10
  - 1,2,3
  - 1-3
  - 5-10,1,3-4

'/<subtest' part is optional, but has the same format. E.g., to select
test #3 and its sub-tests #10 through #15, use: -t 3/10-15.

Similarly, to select tests by name, use `-t verif/strobe`:

  $ sudo ./test_progs -t verif/strobe
  #3/12 strobemeta.o:OK
  #3/13 strobemeta_nounroll1.o:OK
  #3/14 strobemeta_nounroll2.o:OK
  #3 bpf_verif_scale:OK
  Summary: 1/3 PASSED, 0 FAILED

Example of using subtest API is in the next patch, converting
bpf_verif_scale.c tests to use sub-tests.

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