Abstract integration helpers

• I've read the guidelines for Contributing to Roots Projects
• This request isn't a duplicate of an existing request
• This is not a personal support request that should be posted on the Roots Discourse community

Summary

Abstract integration helpers. Something like:

- bin := os.Getenv("TEST_BINARY") 
-  if bin == "" { 
-  	t.Error("TEST_BINARY not supplied") 
-  } 
-  if _, err := os.Stat(bin); os.IsNotExist(err) { 
-  	t.Error(bin + "not exist") 
-  } 
  
-   dummy := os.Getenv("TEST_DUMMY")
- 	if dummy == "" {
- 		t.Error("TEST_DUMMY not supplied")
- 	}

+ bin, dummy = integrationTestHelper.prepareEnvironment()

  // after the tests

+ integrationTestHelper.cleanup(dummy)

integrationTestHelper.prepareEnvironment:

1. Check environment variables are supplied
2. Clone the dummy files into a temporary location (to be deleted by integrationTestHelper.cleanup)
3. return the paths, i.e: bin, dummy

Motivation

Cleaning up becomes problematic when not running tests with docker.

Cloning the dummy files into temporary locations ensure integration tests don't interfere with each others, so that developers don't need to clean up dummy

Cons:

• Tests take longer to run. But I think this is a reasonable trade-off
• premature abstraction is evil

Additional context

https://github.com/roots/trellis-cli/blob/1741735ce5d2362313b247ff526d5a880acabaf1/cmd/dot_env_test.go#L151-L162

Start with cmd/dot_env_test.go, especially the cited lines 151-162 and the cleanup example around line 167. Inspect the integration tests that repeat environment checks and dummy-file setup, then determine the shared helper's scope. Done means tests isolate temporary files, validate required environment variables, and clean up reliably.