cmd/go: show full syntax error list from go/parser

<pre>When one of the test files has a syntax error, I'd expect to see a few lines of error,
as we get when compiling non-test files.

Instead, only a single line of error is printed.
For example:

/home/rog/src/go/src/launchpad.net/goamz/ec2/ec2t_test.go:71:9: expected expression (and
120 more errors)</pre>
