mantle/kola/testiso: add comment about broken test

coreos#3554

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mantle/cmd/kola/testiso.go

@@ -221,6 +221,8 @@ ExecStart=/bin/bash -c '[[ $(findmnt -nvro SOURCE /sysroot) == /dev/mapper/mpath
 [Install]
 RequiredBy=multi-user.target`
 
+// This test is broken. Please fix!
+// https://github.com/coreos/coreos-assembler/issues/3554
 var verifyNoEFIBootEntry = `[Unit]
 Description=TestISO Verify No EFI Boot Entry
 OnFailure=emergency.target