torture: Abstract application of additional Kconfig options

This commit introduces a config_override_param() bash function that
folds in an additional set of Kconfig options.  This is initially applied
to fold in the --kconfig kvm.sh parameter, but later commits will also
apply it to the Kconfig options added by the --kcsan kvm.sh parameter.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>