cmd/compile/internal/syntax: silence test function output

cmd/compile/internal/syntax: silence test function output

Don't print to stdout in non-verbose (-v) test mode.

Exception: Timing output (2 lines) of TestStdLib. If
we want to disable that as well we should use another
flag to differenciate between -verbose output and
measurement results. Leaving alone for now.

Fixes #35223.

Change-Id: Ie8160760e8db1138f9031888d654eaeab202128c
Reviewed-on: https://go-review.googlesource.com/c/go/+/204039
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>