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/experimentinputprocessor.go

@@ -0,0 +1,12 @@
+package mocks
+
+import "context"
+
+// ExperimentInputProcessor processes inputs running the given experiment.
+type ExperimentInputProcessor struct {
+	MockRun func(ctx context.Context) error
+}
+
+func (eip *ExperimentInputProcessor) Run(ctx context.Context) error {
+	return eip.MockRun(ctx)
+}