[podautoscaler]: add more unit test in podautoscaler controller

[googs1025]

for now, podautoscaler controller (pkg/controller/podautoscaler) currently lacks sufficient unit test coverage. This leads to potential regressions when new features or refactors are introduced.
In addition, we are currently hoping that podautoscaler can reach the production stage. #1422

This is important to us.

• Please use table driven approach to implement test cases
• Please ping @Jeffwan or @googs1025 for review

[googs1025]

(vllm) ➜  aibrix git:(fix/hpa_resource) go test -cover ./pkg/controller/podautoscaler/...
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler             coverage: 0.0% of statements
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler/aggregation         coverage: 0.0% of statements
ok      github.com/vllm-project/aibrix/pkg/controller/podautoscaler/algorithm   1.449s  coverage: 41.2% of statements
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler/context             coverage: 0.0% of statements
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler/metrics             coverage: 0.0% of statements
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler/monitor             coverage: 0.0% of statements
        github.com/vllm-project/aibrix/pkg/controller/podautoscaler/types               coverage: 0.0% of statements

[nurali-techie]

/assign

let me give a try 👍

[googs1025]

Note: This is a large workload, so we can split into different PRs based on different dimensions. For example, submitting based on the go file dimension (just a reference).

[nurali-techie]

Sure, I will raise PR mostly one file at a time. If need be then I will give heads up on which file I am wrting unit_tests so there is no overlap if others are taking up unit_tests for other files.