TEST-18-FAILUREACTION: Set auto firmware

This test runs in nspawn by default but will still run in qemu when
tests are run unprivileged so make sure we use UEFI if available to
avoid hangs when using the linux firmware.

(cherry picked from commit f392be9e77)

test/TEST-18-FAILUREACTION/meson.build

@@ -3,5 +3,8 @@
 integration_tests += [
         integration_test_template + {
                 'name' : fs.name(meson.current_source_dir()),
+                # FIXME: Figure out why reboot sometimes hangs with 'linux' firmware.
+                # Use 'auto' to automatically fallback on non-uefi architectures.
+                'firmware' : 'auto',
         },
 ]