koordlet: add page cache collector unit test and mend default cold memory config #6699

Job	Run time
golangci-lint	6m 26s
unit-tests(Check Go modules)	59s
unit-tests(Check manifests)	1m 16s
unit-tests(Check auto-generated codes)	4m 52s
unit-tests(Verify gofmt)	55s
unit-tests(Verify govet)	4m 43s
unit-tests(Run Go build)	3m 44s
unit-tests(Run Go test)	5m 22s
build-and-push	0s
	28m 17s

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