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_test.go

@@ -0,0 +1,22 @@
+package mocks
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestExperimentInputProcessor(t *testing.T) {
+	t.Run("Run", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		eip := &ExperimentInputProcessor{
+			MockRun: func(ctx context.Context) error {
+				return expected
+			},
+		}
+		err := eip.Run(context.Background())
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected result")
+		}
+	})
+}