fix(testutil): fix tracing span collection (#324)

collect all spans if export spans is called more than once

testutil/tracing.go

@@ -25,7 +25,7 @@ type Collector struct {
 
 // ExportSpans receives the ReadOnlySpans from the batch provider
 func (c *Collector) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) error {
-	c.Spans = tracetest.SpanStubsFromReadOnlySpans(spans)
+	c.Spans = append(c.Spans, tracetest.SpanStubsFromReadOnlySpans(spans)...)
 	sort.SliceStable(c.Spans, func(i, j int) bool {
 		return c.Spans[i].StartTime.Before(c.Spans[j].StartTime)
 	})