support/testing/test_hardening: add missing Kconfig symbol

BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y is needed to use the
custom external toolchain x86-i686--glibc--bleeding-edge-2018.11-1.tar.bz2

Otherwise the symbol BR2_TOOLCHAIN_EXTERNAL_URL is lost.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981738
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981739
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981740
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981741
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981742
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981743



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>