roaring64: fix TestSetAndGetBigTimestamp (non-UTC)

The test was broken if local timezone it not UTC. For example, in timezone
UTC-03:00 it failed with this error:

--- FAIL: TestSetAndGetBigTimestamp (0.00s)
    bsi64_test.go:111:
      Error Trace:    bsi64_test.go:111
      Error:          Not equal:
         expected: "3024-10-23T16:55:46.763295273Z"
         actual  : "3024-10-23T13:55:46.763295273-03:00"

Author: starius

roaring64/bsi64_test.go

@@ -108,7 +108,7 @@ func TestSetAndGetBigTimestamp(t *testing.T) {
 	bv, _ := bsi.GetBigValue(1)
 	seconds, nanoseconds := bigIntToSecondsAndNanos(bv)
 	ts := time.Unix(seconds, int64(nanoseconds))
-	assert.Equal(t, "3024-10-23T16:55:46.763295273Z", ts.Format(time.RFC3339Nano))
+	assert.Equal(t, "3024-10-23T16:55:46.763295273Z", ts.UTC().Format(time.RFC3339Nano))
 	assert.Equal(t, 67, bsi.BitCount())
 }