test/system: Properly bail-out if failed to cache images

The 'die' function is a remnant from times before the system tests
rewrite. It served for writing an error message and then failing
the test. Since the rewrite it is no longer present. Instead, simply
use 'false' in case a caching step fails.

Fallout from da6b6a7

containers#899

test/system/000-setup.bats

@@ -5,8 +5,8 @@ load 'libs/helpers'
 @test "test suite: Setup" {
     _setup_environment
     # Cache the default image for the system
-    _pull_and_cache_distro_image $(get_system_id) $(get_system_version) || die
+    _pull_and_cache_distro_image $(get_system_id) $(get_system_version) || false
     # Cache all images that will be needed during the tests
-    _pull_and_cache_distro_image fedora 32 || die
-    _pull_and_cache_distro_image busybox || die
+    _pull_and_cache_distro_image fedora 32 || false
+    _pull_and_cache_distro_image busybox || false
 }