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/submitter.go

@@ -0,0 +1,17 @@
+package mocks
+
+import (
+	"context"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// Submitter mocks model.Submitter.
+type Submitter struct {
+	MockSubmit func(ctx context.Context, m *model.Measurement) error
+}
+
+// Submit calls MockSubmit
+func (s *Submitter) Submit(ctx context.Context, m *model.Measurement) error {
+	return s.MockSubmit(ctx, m)
+}