tests: Validate ESP-related error in no-ESP failure test

The no-ESP test only checked for a non-zero exit code, which would
also pass if podman itself failed for unrelated reasons. Check that
the output contains "ESP" to confirm the right failure mode.

Assisted-by: OpenCode (Claude Opus 4)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

tmt/tests/booted/test-multi-device-esp.nu

@@ -404,6 +404,9 @@ def test_no_esp_failure [] {
         } | complete)
 
         assert ($result.exit_code != 0) "Expected install to fail with no ESP partitions"
+        # Verify the failure is ESP-related, not an unrelated podman/runtime error
+        let combined = $"($result.stdout)\n($result.stderr)"
+        assert ($combined | str contains "ESP") $"Expected ESP-related error message, got: ($combined | str substring 0..200)"
         print $"Install failed as expected with exit code ($result.exit_code)"
     } catch {|e|
         cleanup $vg_name [$loop1, $loop2] $mountpoint