oeqa/selftest/wic: Ensure initramfs exists for test_iso_image
AssertionError: Command 'wic create mkhybridiso --image-name core-image-minimal -o /var/tmp/wic.oe-selftest/' returned non-zero exit status 1:
ERROR: _exec_cmd: gzip -f -9 -c /var/tmp/wic.oe-selftest/tmp.wic.drhn6edm/initrd.cpio > /var/tmp/wic.oe-selftest/tmp.wic.drhn6edm/initrd.cpio.gz returned '1' instead of 0
output: gzip: /var/tmp/wic.oe-selftest/tmp.wic.drhn6edm/initrd.cpio: No such file or directory
This is because in a clean build directory, the initramfs may not be rebuilt.
Add a call to ensure it is built to avoid the error.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
