Implement finalized test suite for the CLI and configuration code

internal/cli/contract_test_helpers_test.go

@@ -0,0 +1,14 @@
+package cli
+
+import (
+	"strings"
+	"testing"
+)
+
+func replaceRequiredOnce(t *testing.T, input, old, replacement string) string {
+	t.Helper()
+	if count := strings.Count(input, old); count != 1 {
+		t.Fatalf("replacement marker %q occurs %d times, want exactly once", old, count)
+	}
+	return strings.Replace(input, old, replacement, 1)
+}