selftests/bpf: Fix test_progs-flavor run getting number of tests

Commit 643e7233 ("selftests/bpf: Test_progs option for getting number of
tests") introduced ability to getting number of tests, which is targeted
towards scripting.  As demonstrate in the commit the number can be use as a
shell variable for further scripting.

The test_progs program support "flavor", which is detected by the binary
have a "-flavor" in the executable name. One example is test_progs-no_alu32,
which load bpf-progs compiled with disabled alu32, located in dir 'no_alu32/'.

The problem is that invoking a "flavor" binary prints to stdout e.g.:
 "Switching to flavor 'no_alu32' subdirectory..."
Thus, intermixing with the number of tests, making it unusable for scripting.

Fix the issue by only printing "flavor" info when verbose -v option is used.

Fixes: 643e7233

 ("selftests/bpf: Test_progs option for getting number of tests")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/159827024012.923543.7104106594870150597.stgit@firesoul