TrafficSegment tests perform 10k test requests per test case.

Add `inlineContent("")` to test routes to have `Content-Length: 0`
response header that disables chunked encoding and speeds up the tests.

Before:
```
go test ./predicates/traffic/ -count=1 -run=TestTrafficSegment
ok      github.com/zalando/skipper/predicates/traffic   8.928s
```

After:
```
go test ./predicates/traffic/ -count=1 -run=TestTrafficSegment
ok      github.com/zalando/skipper/predicates/traffic   4.285s
```

To further speedup make test requests in parallel:
```
go test ./predicates/traffic/ -count=1 -run=TestTrafficSegment
ok      github.com/zalando/skipper/predicates/traffic   1.554s
```

See related #2459 #1562

Signed-off-by: Alexander Yastrebov <[email protected]>