tests: fix flaky SDK assertion

[pmalek]

What this PR does / why we need it:

This PR aims to fix the flakiness in plugin SDK assertions that appeared in https://github.com/Kong/gateway-operator/actions/runs/11097835652/job/30829655627?pr=667#step:5:373

  kongpluginbinding_unmanaged_test.go:104: FAIL:	CreatePlugin(string,string,string)
        		at: [/home/runner/work/gateway-operator/gateway-operator/controller/konnect/ops/plugin_mock.go:76 /home/runner/work/gateway-operator/gateway-operator/test/envtest/kongpluginbinding_unmanaged_test.go:68]
    kongpluginbinding_unmanaged_test.go:104: FAIL: 0 out of 1 expectation(s) were met.
        	The code you are testing needs to make 1 more call(s).
        	at: [/home/runner/work/gateway-operator/gateway-operator/test/envtest/kongpluginbinding_unmanaged_test.go:104 /home/runner/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.5.linux-amd64/src/runtime/asm_amd64.s:1695]

The idea behind the fix is to wait for the resource to have a Konnect ID assigned and only then check the plugin sdk assertions.