test/imgtestlib: skip network-installer boot test on 10.3

The beta repos are showing the same behaviour that was discovered before
with 10.1 and 10.2.

#2044 (comment)

Author: achilleas-k

test/scripts/imgtestlib/core.py

@@ -402,7 +402,7 @@ def can_boot_test(manifest_fname, manifest_data, image_type, arch, distro, bluep
             return False
 
     if image_type in ["network-installer", "everything-network-installer", "server-network-installer"]:
-        if distro in ["rhel-10.1", "rhel-10.2"]:
+        if distro in ["rhel-10.1", "rhel-10.2", "rhel-10.3"]:
             print("  not bootable: rhel network-installer tests have incomplete repos in nightly snapshot"
                   "and won't install")
             return False