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`: