test: fix test names

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This MR adds the missing Test prefix to several functions in _test.go files, so the go test command correctly recognizes them as test functions.

Before (the command counts tests):

$ go test -list . | grep '^Test' | wc -l
    1078

After:

$ go test -list . | grep '^Test' | wc -l
    1088

These were identified with the help of the revive.get-return linter, which I’ve enabled as well

$ golangci-lint run
cluster_agents_test.go:112:1: get-return: function 'GetClusterAgent' seems to be a getter but it does not return any result (revive)
cluster_agents_test.go:272:1: get-return: function 'GetAgentToken' seems to be a getter but it does not return any result (revive)
resource_milestone_events_test.go:74:1: get-return: function 'GetIssueMilestoneEvent' seems to be a getter but it does not return any result (revive)
resource_milestone_events_test.go:198:1: get-return: function 'GetMergeRequestMilestoneEvent' seems to be a getter but it does not return any result (revive)
4 issues:
* revive: 4