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.go

@@ -0,0 +1,19 @@
+package mocks
+
+import (
+	"context"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// ExperimentInputLoader mocks model.ExperimentInputLoader
+type ExperimentInputLoader struct {
+	MockLoad func(ctx context.Context) ([]model.OOAPIURLInfo, error)
+}
+
+var _ model.ExperimentInputLoader = &ExperimentInputLoader{}
+
+// Load calls MockLoad
+func (eil *ExperimentInputLoader) Load(ctx context.Context) ([]model.OOAPIURLInfo, error) {
+	return eil.MockLoad(ctx)
+}