tests: Strengthen SELinux runtime check

/sys/fs/selinux will also exist on e.g. Debian containers running on a
Fedora host. We don't see that in CI as that runs on Ubuntu, but it
breaks local runs.

So instead check if the `selinuxenabled` command is available.

tests/test-umockdev-run.vala

@@ -214,7 +214,12 @@ A: size=1048576\n
 
 #if HAVE_SELINUX
     // we may run on a system without SELinux
-    if (FileUtils.test("/sys/fs/selinux", FileTest.EXISTS)) {
+    try {
+        Process.spawn_command_line_sync ("command -v selinuxenabled", null, null, out exit);
+    } catch (SpawnError e) {
+        exit = 1;
+    }
+    if (exit == 0) {
         check_program_out("true", "-d " + umockdev_file + " -- stat -c %C /dev/loop23",
                           "system_u:object_r:fixed_disk_device_t:s0\n");
     } else {
@@ -352,7 +357,12 @@ t_run_record_null ()
 
 #if HAVE_SELINUX
     // we may run on a system without SELinux
-    if (FileUtils.test("/sys/fs/selinux", FileTest.EXISTS)) {
+    try {
+        Process.spawn_command_line_sync ("command -v selinuxenabled", null, null, out exit);
+    } catch (SpawnError e) {
+        exit = 1;
+    }
+    if (exit == 0) {
         string orig_context;
         assert_cmpint (Selinux.lgetfilecon ("/dev/null", out orig_context), CompareOperator.GT, 0);
         check_program_out("true", "-d " + umockdev_file + " -- stat -c %C /dev/null", orig_context + "\n");

tests/test-umockdev-vala.vala

@@ -203,7 +203,13 @@ t_testbed_fs_ops ()
 void
 t_testbed_selinux ()
 {
-  if (!FileUtils.test("/sys/fs/selinux", FileTest.EXISTS)) {
+  int exit;
+  try {
+      Process.spawn_command_line_sync ("command -v selinuxenabled", null, null, out exit);
+  } catch (SpawnError e) {
+      exit = 1;
+  }
+  if (exit != 0) {
       stdout.printf ("[SKIP SELinux not active]\n");
       return;
   }