REG-TESTS: http-messaging: Send one request per h2 client in h2_trailers.vtc

REG-TESTS: http-messaging: Send one request per h2 client in h2_trailers.vtc

The script fails randomly depending on the load on the machine. C3 and C4 H2
clients are sending sending several invalid requests. However, instead of
received the expected RST_STREAM for some requests, a WINDOW_UPDATE is
received, making the script failed.

So, to avoid this issue, the clients are splitted to send only one h2
request per connection. It seems better this way.