testing: `go test -run` should warn user if there are no tests to run

[gopherbot]

by guohouzuo:

`go test` generates a very useful warning when there are no tests to run:

    > go test
    testing: warning: no tests to run
    PASS
    ok  mypkg  0.010s

However, the behaviour is different when `-run` flag is involved:

    > go test -run=DoesNotExist
    PASS
    ok   mypkg   0.011s

It will be really useful to suggest user of his/her mistake in the second case (with
-run flag).

Thanks.

[minux]

Comment 1:

https://golang.org/cl/90900043

Labels changed: added release-go1.4, repo-main.

Owner changed to @minux.

Status changed to Accepted.

[nathany]

Comment 2:

Just ran into this too.
I discovered this on a test file named *_test_darwin.go instead of *_darwin_test.go. It
seemed like it was working (PASS), but that test wasn't running at all.

[rsc]

Comment 3:

I use things like 'go test -run=XXX' all the time. I don't really want a warning there.
I think this is fine as is.

Owner changed to ---.

Status changed to WorkingAsIntended.