I can’t wait to get all of these automatically from a single source :)

Also change grpcio-tools to be a runtime dependency of grpcio-tests,
grpcio-tools is run as part of the tests.

modified condition for trailing metadata.
added more information to log message.

On my glibc (Debian Jessie amd64 if anybody cares) system,
<linux/unistd.h> is a strict subset of <sys/sycall.h>, which the file is
already including. musl libc doesn't provide this file, and with this
change all the C++ tests pass with musl libc.

This came up in bazelbuild/bazel#1492.