fix: Fix needs to work for upgrades and when selectors are provided (#922)

* fix: Fix `needs` to work for upgrades and when selectors are provided

Fixes #919

* Add test framework for `helmfile apply`

* Various enhancements and fixes to the DAG support

- Make the order of upgrades/deletes more deterministic for testability
- Fix the test framework so that we can validate log outputs and errors
- Add more test cases for `helmfile apply`, along with bug fixes.
- Make sure it fails with an intuitive error when you have non-existent releases referenced from witin "needs"

pkg/helmexec/exit_error.go

@@ -8,8 +8,8 @@ import (
 
 func newExitError(helmCmdPath string, exitStatus int, errorMessage string) ExitError {
 	return ExitError{
-		msg:        fmt.Sprintf("%s exited with status %d:\n%s", filepath.Base(helmCmdPath), exitStatus, indent(strings.TrimSpace(errorMessage))),
-		exitStatus: exitStatus,
+		Message: fmt.Sprintf("%s exited with status %d:\n%s", filepath.Base(helmCmdPath), exitStatus, indent(strings.TrimSpace(errorMessage))),
+		Code:    exitStatus,
 	}
 }
 
@@ -23,14 +23,14 @@ func indent(text string) string {
 
 // ExitError is created whenever your shell command exits with a non-zero exit status
 type ExitError struct {
-	msg        string
-	exitStatus int
+	Message string
+	Code    int
 }
 
 func (e ExitError) Error() string {
-	return e.msg
+	return e.Message
 }
 
 func (e ExitError) ExitStatus() int {
-	return e.exitStatus
+	return e.Code
 }