testing: Improve "testing" package comment for beginners

[cgay]

Go version: 1.2

The Overview section of the "testing" package docs could benefit from
including this line:

```You write a test by creating a file with a name ending in _test.go that contains
functions named TestXXX with signature func (t *testing.T). The test framework runs each
such function; if the function calls a failure function such as t.Error or t.Fail, the
test is considered to have failed.```

That would help beginners get their bearings. I eventually found that explanation here:
http://golang.org/doc/code.html#Testing

While writing my first tests I was looking around for something like assertEqual et al,
and that would have saved me a little time.

Thanks.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.3, documentation.

[robpike]

Comment 2:

This issue was closed by revision f1e4184.

Status changed to Fixed.