cmd/go: runtime/debug build information not populated for test binaries when package name is not 'main'

[bcmills]

What did you do?

https://play.golang.org/p/0ITNcl4kXGN

$ gotip version
go version devel +8c5de667 Fri Aug 30 08:28:40 2019 +0000 linux/amd64

foo$ gotip mod init foo
go: creating new go.mod: module foo

foo$ cat > foo_test.go
package foo_test

import (
        "runtime/debug"
        "testing"
)

func TestBuildInfo(t *testing.T) {
        info, ok := debug.ReadBuildInfo()
        if !ok {
                t.Fatal("no debug info")
        }
        t.Log(info.Main.Version)
}

foo$ go test

What did you expect to see?

=== RUN   TestBuildInfo
--- PASS: TestBuildInfo (0.00s)
    foo_test.go:13: (devel)
PASS

(https://play.golang.org/p/0ITNcl4kXGN)

What did you see instead?

--- FAIL: TestBuildInfo (0.00s)
    foo_test.go:11: no debug info
FAIL

CC @jayconrod @rsc; see also #33975.