Empty the default KERNELS map and make tests chroot-safe

Custom kernels are now set only in the config, so the defaults suit any system. The reboot check's modules folder can be overridden with UPDATE_NOTIFY_MODULES_DIR, so the tests no longer need an installed kernel.

config.example

@@ -9,13 +9,12 @@
 # Name of the local aurutils repository.
 #AUR_REPO="aur-local"
 
-# Stock kernel package -> your custom build of it. Replaces the whole list;
-# kernels where either package isn't installed are skipped.
-# Set KERNELS=() to disable the kernel check.
+# Stock kernel package -> your custom build of it, to be reminded to rebuild
+# it. Empty by default (no kernel check); kernels where either package isn't
+# installed are skipped. Example:
 #KERNELS=(
 #    [linux]="linux-custom"
 #    [linux-lts]="linux-lts-custom"
-#    [linux-zen]="linux-zen-custom"
 #)
 
 # Remind about .pacnew/.pacsave files to merge with pacdiff (0 to disable).