test: Check os-release instead of /dev/kvm for Machines tests

Commit e17149cb did not work as intended -- the RHEL gating machines
also don't have /dev/kvm, but are still powerful enough to run all
Machines tests. So consult /etc/os-release instead of /dev/kvm.

tests/run-test.sh

@@ -53,10 +53,10 @@ if [ -n "$test_optional" ]; then
          TestUpdates.testSecurityOnly"
 
     # Fedora gating tests are running on infra without /dev/kvm; Machines tests are too darn slow there
-    if [ -w /dev/kvm ]; then
-        TESTS="$TESTS TestMachines"
-    else
+    if [ "$ID" = "fedora" ]; then
         TESTS="$TESTS TestMachinesCreate.testCreateImportDisk"
+    else
+        TESTS="$TESTS TestMachines"
     fi
 fi