test: Fix flaky receive/multitsdb test (thanos-io#7694)

There is race condition in `TestMultiTSDBPrune` due to a dangling goroutine
which can fail outside of the test function's lifetime if the database object
is closed before `Sync()` is finished.

Signed-off-by: Mikhail Nozdrachev <mikhail.nozdrachev@aiven.io>

pkg/receive/multitsdb_test.go

@@ -462,10 +462,14 @@ func TestMultiTSDBPrune(t *testing.T) {
 			testutil.Equals(t, 3, len(m.TSDBLocalClients()))
 
 			ctx, cancel := context.WithCancel(context.Background())
-			defer cancel()
+
+			g := sync.WaitGroup{}
+			defer func() { cancel(); g.Wait() }()
 
 			if test.bucket != nil {
+				g.Add(1)
 				go func() {
+					defer g.Done()
 					testutil.Ok(t, syncTSDBs(ctx, m, 10*time.Millisecond))
 				}()
 			}