🌱 test: fix machinepool test to wait for topology controller to set correct number of replicas first

[chrischdi]

What this PR does / why we need it:

I did hit this locally:
I had a case where 3 healthy machine pool machines did exist, but the test expected two while it must be three.

The topology controller may require some time to rollout the change to the MachinePool object.
This ensures we get the right state of the machine pool object to continue checking the right amount of ready replicas.

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 #

/area e2e-testing

[sbueringer]

Thx!

/lgtm
/approve

/test pull-cluster-api-e2e-main

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 0a77809f988f843fec654c4955a6b43b11c414e6

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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[chrischdi]

/retest

Webhook flake, unrelated

[chrischdi]

/cherry-pick release-1.7

[k8s-infra-cherrypick-robot]

@chrischdi: #10952 failed to apply on top of branch "release-1.7":

Applying: test: fix machinepool test to wait for topology controller to set correct number of replicas first
Using index info to reconstruct a base tree...
M	test/framework/machinepool_helpers.go
M	test/infrastructure/docker/exp/internal/controllers/dockermachinepool_controller.go
Falling back to patching base and 3-way merge...
Auto-merging test/infrastructure/docker/exp/internal/controllers/dockermachinepool_controller.go
Auto-merging test/framework/machinepool_helpers.go
CONFLICT (content): Merge conflict in test/framework/machinepool_helpers.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 test: fix machinepool test to wait for topology controller to set correct number of replicas first
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-1.7

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