Rework restart-and-reload to assert more strictly on the activation output

I've reworked the test to assert on the entire output. This allows us to
detect unexpected output without having to write weird "i expect this
random string to *not* show up assertions", which aren't great at
preventing regressions.

I did have to change the code under test a little bit to make it
behavior deterministically (by sorting the files it outputs).

tl;dr: this demonstrates <https://github.com/Mic92/sops-nix/issues/652>
but does not fix it. I will fix it in a subsequent commit.

pkgs/sops-install-secrets/main.go

@@ -11,6 +11,7 @@ import (
 	"os/user"
 	"path"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 	"syscall"
@@ -984,12 +985,15 @@ func handleModifications(isDry bool, logcfg loggingConfig, symlinkPath string, s
 			} else {
 				fmt.Printf("%s secret%s: ", regularPrefix, s)
 			}
-			comma := ""
-			for name := range changed {
-				fmt.Printf("%s%s", comma, name)
-				comma = ", "
+
+			// Sort the output for deterministic behavior.
+			keys := make([]string, 0, len(changed))
+			for key := range changed {
+				keys = append(keys, key)
 			}
-			fmt.Println()
+			sort.Strings(keys)
+
+			fmt.Println(strings.Join(keys, ", "))
 		}
 	}
 	outputChanged(newSecrets, "adding", "would add")