tests/kern: make ssl_sendfile:truncate and ssl_sendfile:grow less flaky

First problem is a trivial race that the client thread doesn't see updated
c.sbytes. Second problem applies only to the trunc

tests/kern: make ssl_sendfile:truncate and ssl_sendfile:grow less flaky

First problem is a trivial race that the client thread doesn't see updated
c.sbytes. Second problem applies only to the truncate test. On a machine
with huge default buffer sizes, there is a chance that sendfile(2) will
fill both buffers with amount of data that is larger than the size we plan
to truncate. To minimise chances for this scenario, increase file size
and truncate it less aggressively, also try to decrease buffer sizes.

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tests/kern: add test for SSL_sendfile()