Refactor MachinePool unit tests to use Builder/Options

[2uasimojo]

In preparation for expanding OpenStack-specific MachinePool unit tests, refactor the helper funcs in the existing suite to use the MachinePool "Builder" pattern with "Options". For example, we change this:

machinePool: func() *hivev1.MachinePool {
        mp := testMachinePoolWithoutLabelsTaints()
        mp.Spec.Labels = make(map[string]string)
        mp.Spec.Labels["test-label"] = "test-value"
        return mp
}(),

to:

machinePool: testMachinePoolWithoutLabelsTaints(
        testmp.WithLabels(map[string]string{
                "test-label": "test-value",
        }),
),

This makes the test code slightly tighter and more readable, and brings the suite more in line with our usual (or at least, best practice) patterns.

Facilitates HIVE-2476

[2uasimojo]

NB: Currently stacked on #2251

[openshift-ci]

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[2uasimojo]

/assign @suhanime

(Just look at the last commit please -- @lleshchi is reviewing the rest via #2251.)
[Later] Rebased; just the one commit to look at now.

[codecov]

Codecov Report

Attention: Patch coverage is 94.20290% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 58.58%. Comparing base (5f27f3c) to head (9a5473a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2252      +/-   ##
==========================================
+ Coverage   58.47%   58.58%   +0.11%     
==========================================
  Files         182      182              
  Lines       25719    25775      +56     
==========================================
+ Hits        15038    15101      +63     
+ Misses       9411     9402       -9     
- Partials     1270     1272       +2     

Files	Coverage Δ
pkg/test/machinepool/machinepool.go	94.17% <94.20%> (+21.83%)	⬆️

[2uasimojo]

/test e2e-gcp

[2uasimojo]

/test e2e-gcp

[suhanime]

/lgtm