I've run into a few cases where running the full test suite for the ARM
builder would fail because of conflicting image names. This is a
workaround for now the bigger fix is to have the acceptance clean up any
created images.
Results before change
```
the managed image named testBuilderAccManagedDiskWindows-1603151855 already exists in the resource group packer-acceptance-test, use the -force option to automatically delete it.
--- FAIL: TestBuilderAcc_ManagedDisk_Windows_Build_Resource_Group (0.66s)
```
Results after change
```
PASS
ok github.com/hashicorp/packer/builder/azure/arm 2431.115s
```
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