Resolve architect findings: gate [ on json.Valid, fix the root-relative README claim, test isTemplate

cmd/fejkdata/main_test.go

@@ -301,6 +301,23 @@ func TestRunShippedDataByDefault(t *testing.T) {
 	}
 }
 
+func TestIsTemplate(t *testing.T) {
+	for arg, want := range map[string]bool{
+		"sv_SE.person":      false,
+		"person.last":       false,
+		"[abc]":             false, // a [ can open a real category name; not JSON
+		"[abc].field":       false,
+		"name: {x}":         true, // a { token: a path can never carry a brace
+		`{"format":"x"}`:    true,
+		`["a","b"]`:         true, // a JSON array carries no brace
+		`[1, 2]`:            true,
+	} {
+		if got := isTemplate(arg); got != want {
+			t.Errorf("isTemplate(%q) = %v, want %v", arg, got, want)
+		}
+	}
+}
+
 func TestRunInlineTemplate(t *testing.T) {
 	code, out, errb := runOut("--seed", "1", "name: {/sv_SE.person.last}")
 	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {