clean up scope unit tests

[nojnhuh]

What type of PR is this?

What this PR does / why we need it:

This PR removes most usage of our constructor wrappers to create Scope objects in unit tests in order to avoid the extra setup that requires to create a valid reference to an existing AzureClusterIdentity. That adds quite a few moving parts when all we are trying to test is a function like this that doesn't actually use the identity at all:

cluster-api-provider-azure/azure/scope/cluster.go

Lines 636 to 645 in b8f23bd

	// Subnet returns the subnet with the provided name.
	func (s *ClusterScope) Subnet(name string) infrav1.SubnetSpec {
	for _, sn := range s.AzureCluster.Spec.NetworkSpec.Subnets {
	if sn.Name == name {
	return sn
	}
	}
	return infrav1.SubnetSpec{}
	}

I'm planning to make some changes to the identity stuff soon for #5017 and #1077 that would require some change to all of these tests, but with this change it shouldn't need any extra changes. Hopefully this prevents us from needing other sweeping changes to these tests for unrelated changes like this:

https://github.com/kubernetes-sigs/cluster-api-provider-azure/pull/4208/files#diff-bae329736fc788cebbe920f1680bfd39eb52e6f1bdaaa0a620c60037d3840462

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.25%. Comparing base (9eaaadc) to head (a752b27).
Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5143   +/-   ##
=======================================
  Coverage   51.25%   51.25%           
=======================================
  Files         273      273           
  Lines       24651    24651           
=======================================
  Hits        12636    12636           
  Misses      11229    11229           
  Partials      786      786           

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[mboersma]

/retest

[mboersma]

/lgtm

[mboersma]

I guess we can ditch this now with Go 1.22?

[nojnhuh]

I think this change was probably safe even before that since these tests aren't running in parallel.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 186123818041bf58a5cfccde1b627792b2646105

[mboersma]

/assign @willie-yao

[willie-yao]

/approve

[k8s-ci-robot]

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