features: add basic feature flag implementation

internal/feature/testing.go

@@ -0,0 +1,29 @@
+package feature
+
+import (
+	"fmt"
+	"testing"
+)
+
+// TestSetFlag temporarily sets a feature flag to the given value until the
+// returned function is called.
+//
+// Usage
+// ```
+// defer TestSetFlag(t, features.Flags, features.ExampleFlag, true)()
+// ```
+func TestSetFlag(t *testing.T, f *FlagSet, flag FlagName, value bool) func() {
+	current := f.Enabled(flag)
+
+	if err := f.Apply(fmt.Sprintf("%s=%v", flag, value)); err != nil {
+		// not reachable
+		panic(err)
+	}
+
+	return func() {
+		if err := f.Apply(fmt.Sprintf("%s=%v", flag, current)); err != nil {
+			// not reachable
+			panic(err)
+		}
+	}
+}