feat(oonirun): improve tests (#915)

See https://github.com/ooni/probe/issues/2184

While there, rename `runtimex.PanicIfFalse` to `runtimex.Assert` (it was about time...)

internal/model/mocks/experimentinputloader_test.go

@@ -0,0 +1,27 @@
+package mocks
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func TestExperimentInputLoader(t *testing.T) {
+	t.Run("Load", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		eil := &ExperimentInputLoader{
+			MockLoad: func(ctx context.Context) ([]model.OOAPIURLInfo, error) {
+				return nil, expected
+			},
+		}
+		out, err := eil.Load(context.Background())
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if len(out) > 0 {
+			t.Fatal("unexpected length")
+		}
+	})
+}