Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/backup/exectest_test.go

@@ -0,0 +1,48 @@
+package backup
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// withFakeExecutable puts a fake executable named `name` at the front of
+// PATH for the duration of the test, so code that shells out to a
+// real-world tool (pg_dump, mysqldump, fdbbackup, vandelay, python3) can be
+// exercised without that tool actually being installed. t.Setenv restores
+// PATH automatically and marks the test non-parallel.
+func withFakeExecutable(t *testing.T, name, script string) (dir string) {
+	t.Helper()
+	dir = t.TempDir()
+	path := filepath.Join(dir, name)
+	if err := os.WriteFile(path, []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", dir+string(os.PathListSeparator)+os.Getenv("PATH"))
+	return dir
+}
+
+// argsFile is a shared convention the fake scripts below use: they append
+// their own argv (space-joined, one invocation per line) to a file so the
+// test can assert on exactly what was passed.
+func argsFile(t *testing.T, dir string) string {
+	t.Helper()
+	return filepath.Join(dir, "invoked-args.log")
+}
+
+func readArgsFile(t *testing.T, path string) string {
+	t.Helper()
+	data, err := os.ReadFile(path)
+	if os.IsNotExist(err) {
+		return ""
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(data)
+}
+
+func fakeScriptLoggingArgs(logPath string, extraBody string) string {
+	return fmt.Sprintf("#!/bin/sh\necho \"$@\" >> %q\n%s\n", logPath, extraBody)
+}