cmd/vet: add a check for tests with malformed names

cmd/vet: add a check for tests with malformed names

According to golang.org/pkg/testing the first character after Test has
to be non-lowercase. Functions that don't conform to this are not
considered tests and are not loaded which can cause surprises.

This change adds a check to warn about Test-like functions in a _test
file that are not actually run by go test.

Moved over from https://go-review.googlesource.com/#/c/19466/

Change-Id: I2f89676058b27a0e35f721bdabc9fa8a9d34430d
Reviewed-on: https://go-review.googlesource.com/19724
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>