test: PC-13893 Make E2E tests forward compatible (#551)

## Motivation

`value` field of `objective` for Composite SLO is being
[deprecated](#549). Before the
update in SDK can be made, a N9 platform first needs to be
[updated](nobl9/n9#15406). To be able to release
changes in the platform, e2e tests of the latest released SDK need to
pass against it.

## Summary

E2E test assertions will pass with and without
[changes](nobl9/n9#15406) in N9 platform.

## Related changes

#551
#549
nobl9/n9#15406
nobl9/terraform-provider-nobl9#312
nobl9/terraform-provider-nobl9#295

## Testing

Run `make test/e2e` on both environments containing and not containing
nobl9/n9#15406.

tests/v1alpha_slo_test.go

@@ -293,5 +293,8 @@ func assertV1alphaSLOsAreEqual(t *testing.T, expected, actual v1alphaSLO.SLO) {
 	actual.Spec.CreatedBy = ""
 	actual.Status = nil
 	actual.Spec.TimeWindows[0].Period = nil
+	if actual.Spec.Objectives[0].Value == nil {
+		actual.Spec.Objectives[0].Value = ptr(0.0)
+	}
 	assert.Equal(t, expected, actual)
 }