selftests/bpf: Fix cd_flavor_subdir() of test_progs

Currently, when we run test_progs with just executable file name, for
example 'PATH=. test_progs-no_alu32', cd_flavor_subdir() will not check
if test_progs is running as a flavored test runner and switch into
corresponding sub-directory.

This will cause test_progs-no_alu32 executed by the
'PATH=. test_progs-no_alu32' command to run in the wrong directory and
load the wrong BPF objects.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220403135245.1713283-1-ytcoode@gmail.com