Change delete CertificateRequest to defer so it is always done

[JoshVanL]

fixes #73

This PR moves the deletion of CertificateRequests to a defer, which means they are always deleted when not preserving, even if the request failed.

We also catch cases in the watcher where the request may have been deleted out of band.

Adds unit tests for the cert-manager package.

/assign @irbekrm

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[irbekrm]

Looks good to me, the new tests make sense. watcher.Modify is interesting 👀

I've added one suggestion, otherwise happy to lgtm!

[JoshVanL]

@irbekrm Please take another look

[irbekrm]

Thanks for the changes, this looks good to me!

/lgtm

[irbekrm]

Maybe the Prowjob needs more resources

/retest

[irbekrm]

/retest

[irbekrm]

/test pull-istio-csr-k8s-v1-21-istio-v1-9

[jetstack-bot]

@JoshVanL: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-istio-csr-k8s-v1-21-istio-v1-9	db1ab5c	link	/test pull-istio-csr-k8s-v1-21-istio-v1-9

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