[chore][processor/cumulativetodeltaprocessor] Enable goleak check (#30804)

**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 the `goleak` check on all tests in this package, to ensure
the processor isn't leaking goroutines. The only update here is in
testing, as one test was missing a necessary `shutdown` call.

**Link to tracking Issue:** <Issue number if applicable>
#30438

**Testing:** <Describe what testing was performed and which tests were
added.>
All tests are passing, including added `goleak` check.

Author: crobert-1

processor/cumulativetodeltaprocessor/factory_test.go

@@ -62,6 +62,7 @@ func TestCreateProcessors(t *testing.T) {
 				consumertest.NewNop())
 			assert.NotNil(t, mp)
 			assert.NoError(t, mErr)
+			assert.NoError(t, mp.Shutdown(context.Background()))
 		})
 	}
 }

processor/cumulativetodeltaprocessor/package_test.go

@@ -0,0 +1,17 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package cumulativetodeltaprocessor
+
+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/census-instrumentation/opencensus-go/issues/1191 for more information.
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m, goleak.IgnoreTopFunction("go.opencensus.io/stats/view.(*worker).start"))
+}