sdk/trace: Fix goroutine leak in simpleSpanProcessor.Shutdown (#6368)

Side-effect when working towards
https://github.com/open-telemetry/opentelemetry-go/issues/6360

- Fix goroutine leaks from tests in places where batch processor was not
shut down
- Fix goroutine leak in simpleSpanProcessor.Shutdown

sdk/trace/util_test.go

@@ -4,12 +4,18 @@
 package trace_test
 
 import (
+	"context"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 )
 
-func basicTracerProvider(_ *testing.T) *sdktrace.TracerProvider {
+func basicTracerProvider(t *testing.T) *sdktrace.TracerProvider {
 	tp := sdktrace.NewTracerProvider(sdktrace.WithSampler(sdktrace.AlwaysSample()))
+	t.Cleanup(func() {
+		assert.NoError(t, tp.Shutdown(context.Background()))
+	})
 	return tp
 }