valgrind: skip flaky ptest fb_test_amd64

Recently, the none/tests/amd64/fb_test_amd64 test had been flaky and
causing failures on the auto-builder. Until we can get to the root cause
of the issue, we are going to skip the test to reduce the noise from the
ptests.

Signed-off-by: Tony Tascioglu <tony.tascioglu@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>