t/integration.t: switch PUT tests to MD5, reuse buffers

t/integration.t: switch PUT tests to MD5, reuse buffers

MD5 is faster, and these tests aren't meant to be secure,
they're just for checking for data corruption.
Furthermore, Content-MD5 is a supported HTTP trailer and
we can verify that here to obsolete other tests.

Furthermore, we can reuse buffers on env['rack.input'].read
calls to avoid malloc(3) and GC overhead.

Combined, these give roughly a 3% speedup for t/integration.t
on my system.