Modify the regex in TestLinuxSendfile to not match the parameters of
the syscall, just its name and the opening parenthesis. This is enough
to recognize that the syscall was invoked.

This fixes the TestLinuxSendfile test when running in Clear Linux,
where strace always execute with -yy implied, having output with extra
information in the parameters:

    [pid  5336] sendfile(6<TCP:[127.0.0.1:35007->127.0.0.1:55170]>, 8</home/c/src/go/src/net/http/testdata/index.html>, NULL, 22) = 22

Change-Id: If7639b785d5fdf65fae8e6149a97a57b06ea981c
Reviewed-on: https://go-review.googlesource.com/85657Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>