Tests: restored HTTP/2 request body tests with Content-Length.

This partially reverts 3a334b20208e, which prevented testing request body
with Content-Length.  Arguably, it appeared as an early attempt to address
new implementation requirements in 0aa07850922f (1.9.15) for request body
flow control.  After the HTTP/2 request body preread buffer introduction
in ce94f07d5082 (1.11.0), and emitting HTTP/2 SETTINGS acknowledgement in
tests since 48d277065309, it became barely necessary, if ever.