Worker: add test flag to enable Blender-dependent test

Add a `-withBlender` CLI argument for a unit test, to aid in debugging
T99438.

Run the test with `go test ./internal/worker/find_blender/ -args -withBlender`
to actually fail when the file association with `.blend` files cannot be
found.

Note that this doesn't rely on Blender being runnable, but it does rely
on _something_ being associated with .blend files.

internal/worker/find_blender/windows_test.go

@@ -4,17 +4,24 @@
 package find_blender
 
 import (
+	"flag"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+var withBlender = flag.Bool("withBlender", false, "run test that requires Blender to be installed")
+
 // TestFindBlender is a "weak" test, which actually accepts both happy and unhappy flows.
 // It would be too fragile to always require a file association to be set up with Blender.
 func TestFindBlender(t *testing.T) {
 	exe, err := FindBlender()
 	if err != nil {
 		assert.Empty(t, exe)
+
+		if *withBlender {
+			t.Fatalf("unexpected error: %v", err)
+		}
 		return
 	}
 	assert.NotEmpty(t, exe)