[chore][exporter/exporterhelper] Enable goleak test (#9215)

**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.-->
This enables `goleak` to run on tests within `exporter/exporterhelper`.
A known leak needs to be ignored, and we need to explicitly call
shutdown on the queue to make sure no routines are still running after
the test is complete.

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

**Testing:** <Describe what testing was performed and which tests were
added.>
Goleak is now passing, all existing tests are passing.

exporter/exporterhelper/package_test.go

@@ -0,0 +1,17 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package exporterhelper
+
+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"))
+}

exporter/exporterhelper/queue_sender_test.go

@@ -325,6 +325,7 @@ func TestQueuedRetryPersistentEnabled_NoDataLossOnShutdown(t *testing.T) {
 		newNoopObsrepSender, WithRetry(rCfg), WithQueue(qCfg))
 	require.NoError(t, err)
 	require.NoError(t, be.Start(context.Background(), host))
+	t.Cleanup(func() { require.NoError(t, be.Shutdown(context.Background())) })
 
 	// wait for the item to be consumed from the queue
 	replacedReq.checkNumRequests(t, 1)