lint: add test case to cover allowed errors.New(fmt.Sprintf()) cases

Updates golang#350

testdata/errorf-custom.go

@@ -0,0 +1,25 @@
+// Test for allowed errors.New(fmt.Sprintf()) when a custom errors package is imported.
+
+// Package foo ...
+package foo
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+)
+
+func f(x int) error {
+	if x > 10 {
+		return errors.New(fmt.Sprintf("something %d", x)) // OK
+	}
+	if x > 5 {
+		return errors.New(g("blah")) // OK
+	}
+	if x > 4 {
+		return errors.New("something else") // OK
+	}
+	return nil
+}
+
+func g(s string) string { return "prefix: " + s }