tests/functional: Fix reverse_debugging asset precaching

tests/functional: Fix reverse_debugging asset precaching

This commit fixes the asset precaching in the reverse_debugging test on
aarch64.

QemuBaseTest.main() precaches assets (kernel, rootfs, DT blobs, etc.)
that are defined in variables with the ASSET_ prefix. This works because
it ultimately calls Asset.precache_test(), which relies on introspection
to locate these variables.

If an asset variable is not named with the ASSET_ prefix, precache_test
cannot find the asset and precaching silently fails. Hence, fix the
asset precaching by fixing the asset variable name.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-ID: <20250827001008.22112-1-gustavo.romero@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>