[chore][receiver/scraperhelper] Enable goleak check (#9226)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
Add `goleak` to detect leaks in tests. Leaking goroutines were detected
that were caused by a dependency that we can ignore
(`go.opencensus.io/stats/view.(*worker).start`), a `Shutdown` call was
also added that was missing.

**Link to tracking Issue:** <Issue number if applicable>
#9165 

**Testing:** <Describe what testing was performed and which tests were
added.>
Added check is passing as well as existing tests.

receiver/scraperhelper/package_test.go

@@ -0,0 +1,17 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package scraperhelper
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+// The IgnoreTopFunction call prevents catching the leak generated by opencensus
+// defaultWorker.Start which at this time is part of the package's init call.
+// See https://github.com/open-telemetry/opentelemetry-collector/issues/9165#issuecomment-1874836336 for more context.
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m, goleak.IgnoreTopFunction("go.opencensus.io/stats/view.(*worker).start"))
+}

receiver/scraperhelper/scrapercontroller_test.go

@@ -351,6 +351,7 @@ func TestSingleScrapePerInterval(t *testing.T) {
 	require.NoError(t, err)
 
 	require.NoError(t, receiver.Start(context.Background(), componenttest.NewNopHost()))
+	defer func() { require.NoError(t, receiver.Shutdown(context.Background())) }()
 
 	tickerCh <- time.Now()