go/analysis/analysistest: refuse to analyze zero packages

It's easy to forget to pass the last argument,
in which case the test would silently pass.

Change-Id: I95249e1fe8bee75cfaa535fcf723d04f102214fc
Reviewed-on: https://go-review.googlesource.com/138395
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/analysistest/analysistest.go

@@ -66,6 +66,9 @@ type Testing interface {
 // You may wish to call this function from within a (*testing.T).Run
 // subtest to ensure that errors have adequate contextual description.
 func Run(t Testing, dir string, a *analysis.Analyzer, pkgnames ...string) {
+	if pkgnames == nil {
+		t.Errorf("Run: no packages")
+	}
 	for _, pkgname := range pkgnames {
 		pkg, err := loadPackage(dir, pkgname)
 		if err != nil {