[BUG] Intermittent failure in TestInputCallbackDangle test

[pwhelan]

Summary

The TestInputCallbackDangle test fails intermittently with a panic on sending to a closed channel. This was introduced in my PR #43 or one of its descendants. For now it is mitigated in CI by retrying the tests several times but it should be fixed.

go test -v ./
=== RUN   TestInputCallbackCtrlC
2023/10/20 12:25:11 goroutine will be stopping: name=""
--- PASS: TestInputCallbackCtrlC (0.00s)
=== RUN   TestInputCallbackDangle
panic: send on closed channel

goroutine 66 [running]:
github.com/calyptia/plugin.testPluginInputCallbackDangle.Collect(...)
	/home/pwhelan/Projects/work/flb/plugins/flb-go-plugin/cshared_test.go:74
github.com/calyptia/plugin.prepareInputCollector.func1.1(0xc0000ac480?)
	/home/pwhelan/Projects/work/flb/plugins/flb-go-plugin/cshared.go:206 +0x3f
created by github.com/calyptia/plugin.prepareInputCollector.func1 in goroutine 50
	/home/pwhelan/Projects/work/flb/plugins/flb-go-plugin/cshared.go:205 +0xfa
FAIL	github.com/calyptia/plugin	0.005s
FAIL