boards: nrf52_bsim: always allow testing with no optimization

Since f5830f3c, running tests with no optimization has to be confirmed
by setting ZTEST_ALLOW_NO_OPTIMIZATIONS. This is normally done on a per
test basis, but since nrf52_bsim disables optimizations in the
defconfig, the default option has to be flipped at board level so that
every test for this board builds correctly.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>