cmd/go: always build package during "go test" command

even when there are no *_test.go files present.
rsc suggested this change

Fixes #7108

LGTM=r, adg
R=golang-codereviews, r, adg
CC=golang-codereviews
https://golang.org/cl/84300043

src/cmd/go/test.bash

@@ -700,6 +700,14 @@ if ! ./testgo list -f "GOARCH: {{context.GOARCH}}"; then
 	ok=false
 fi
 
+TEST 'Issue 7108: cmd/go: "go test" should fail if package does not build'
+export GOPATH=$(pwd)/testdata
+if ./testgo test notest >/dev/null 2>&1; then
+	echo 'go test notest succeeded, but should fail'
+	ok=false
+fi
+unset GOPATH
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/test.go

@@ -524,7 +524,7 @@ func contains(x []string, s string) bool {
 
 func (b *builder) test(p *Package) (buildAction, runAction, printAction *action, err error) {
 	if len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
-		build := &action{p: p}
+		build := b.action(modeBuild, modeBuild, p)
 		run := &action{p: p, deps: []*action{build}}
 		print := &action{f: (*builder).notest, p: p, deps: []*action{run}}
 		return build, run, print, nil

src/cmd/go/testdata/src/notest/hello.go

+package notest
+
+func hello() {
+	println("hello world")
+}
+Hello world