testing/synctest: test log is not printed without -v

This test fails:

```go
func Test(t *testing.T) {
	synctest.Test(t, func(t *testing.T) {
		t.Error("error")
	})
}
```

However, when run without `-v`, it doesn't output the test log:

```
--- FAIL: Test (0.00s)
FAIL
FAIL	_	0.252s
FAIL
```

Obviously, it should.