ktest: Allow initrd processing without modules defined

When a config is set with CONFIG_MODULES=n, it does not mean that the
kernel does not need an initrd to boot. For systems that depend on LVM
and such, an initrd must run first.

If POST_INSTALL is defined, then run the post install regardless if
modules are needed or not.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>