ktest: pass KERNEL_VERSION to POST_KTEST

ktest: pass KERNEL_VERSION to POST_KTEST

For BLS, kernel entry is added by kernel-install command through
POST_INSALL, for example,

POST_INSTALL = ssh root@Test "/usr/bin/kernel-install \
    add $KERNEL_VERSION /boot/vmlinuz-$KERNEL_VERSION"

The entry is removed by kernel-install command and the kernel
version is needed for the argument.

Pass KERNEL_VERSION variable to POST_KTEST so that kernel-install
command can remove the entry like as follows:

POST_KTEST = ssh root@Test "/usr/bin/kernel-install remove $KERNEL_VERSION"

Link: http://lkml.kernel.org/r/20190509213647.6276-5-msys.mizuma@gmail.com
Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>