selftests/bpf: Also print test name in subtest status message

This patch add test name in subtest status message line, making it possible to
grep ':OK' in the output to generate a list of passed test+subtest names, which
can be processed to generate argument list to be used with "-a", "-d" exact
string matching.

Example:

 #1/1 align/mov:OK
 ..
 #1/12 align/pointer variable subtraction:OK
 #1 align:OK

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210817044732.3263066-4-fallentree@fb.com