CI: add testing and testimage

Add a testing task to the gitlab CI and run testimage in that job.
Currently, it only runs against some of the qemu based machines.
Using slirp, there is no need for a privileged container or tun/tap
devices.

Change-Id: Ia85a3d0089f7d4dc7595c3a45d328c79d8e675f1
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/testimage.yml

@@ -0,0 +1,14 @@
+header:
+  version: 9
+
+local_conf_header:
+  testimage: |
+    IMAGE_CLASSES += "testimage"
+  slirp: |
+    TEST_RUNQEMUPARAMS = "slirp"
+    TEST_SERVER_IP = "127.0.0.1"
+    QEMU_USE_SLIRP = "1"
+
+# Multiple targets are available, put it down to just one
+target:
+  - core-image-base