boards: native_posix: Disable default testing on native_posix_64

Enabling of testing/default impacts all PR CIs.  There isn't that much
extra value for all PRs to have both native_posix and native_posix_64.
Go with native_posix, since most targets are 32-bit on Zephyr.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>