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.

(From OE-Core rev: 2a80fa234d31992691a157425e8990db30158fd1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/selftest/cases/wic.py

@@ -191,7 +191,7 @@ class Wic(OESelftestTestCase):
                  'MACHINE_FEATURES_append = " efi"\n'\
                  'DEPENDS_pn-core-image-minimal += "syslinux"\n'
         self.append_config(config)
-        bitbake('core-image-minimal')
+        bitbake('core-image-minimal core-image-minimal-initramfs')
         self.remove_config(config)
         cmd = "wic create mkhybridiso --image-name core-image-minimal -o %s" % self.resultdir
         self.assertEqual(0, runCmd(cmd).status)