Add unit tests for the custom plugin monitor

[DigitalVeer]

This PR bumps the unit test coverage for custom plugin monitor from 0.8% to 95.3%

---

This PR adds tests for generateStatus, the monitor lifecycle and the metrics path.

This is the first item of #1328.

Package	Before	After
pkg/custompluginmonitor	0.8%	95.3%

What the tests check

• TestGenerateStatusForConditions replays plugin results
  against one monitor and checks the condition and the event after each step for five scenarios.

• TestGenerateStatusForTemporaryProblem checks that a temporary rule creates an event and
  moves no condition.

• TestGenerateStatusMetrics checks the problem counter and the problem gauge through the
  existing metrics stub. The counter increments for new problems only.

• TestMonitorLoopReportsPluginResults and TestMonitorLoopExitsWhenResultChannelCloses
  check the goroutine lifecycle. Stop must return. A closed result channel must end the
  loop.

• TestNewCustomPluginMonitorOrDie writes a config file and checks that the constructor
  applies the defaults.

The lifecycle tests use the plugin scripts in pkg/custompluginmonitor/plugin/test-data.

Validation

Check	Result
go test -cover ./pkg/custompluginmonitor/	95.3%
go test -timeout=1m -race -short -count=5	pass, 3.6s
golangci-lint run --config .golangci.yml	0 issues

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