tests/dmesg: create test for --kmsg-file and --json option.

tests/by-util/test_dmesg.rs

@@ -0,0 +1,21 @@
+// This file is part of the uutils coreutils package.
+//
+// For the full copyright and license information, please view the LICENSE
+// file that was distributed with this source code.
+use crate::common::util::TestScenario;
+
+#[test]
+fn test_invalid_arg() {
+    new_ucmd!().arg("--definitely-invalid").fails().code_is(1);
+}
+
+#[test]
+fn test_kmsg_json() {
+    new_ucmd!()
+        .arg("--kmsg-file")
+        .arg("kmsg.input")
+        .arg("--json")
+        .run()
+        .no_stderr()
+        .stdout_is_fixture("test_kmsg_json.expected");
+}