test/system: Fix reading the os-release(5) ID on Ubuntu

The current approach of selecting all the os-release(5) fields that have
'ID' in their name (eg., ID, VERSION_ID, PLATFORM_ID, VARIANT_ID, etc.)
and then picking the first one, assumes that the ID field will always be
placed above the others in os-release(5).  There's no guarantee that
this will be the case.  It only happens to be so on Fedora by chance,
and is different on Ubuntu:
  $ cat /etc/os-release
  ...
  VERSION_ID="22.04"
  ...
  ID=ubuntu
  ID_LIKE=debian
  ...

This means that "22.04" is read as the value of ID on Ubuntu, which is
clearly wrong.

Instead, use the same approach as profile.d/toolbox.sh and the old POSIX
shell implementation that doesn't rely on the order of the os-release(5)
fields.

Fallout from 54a2ca1

containers#1320

test/system/libs/helpers.bash

@@ -494,7 +494,7 @@ function find_os_release() {
 
 
 # Returns the content of field ID in os-release
-function get_system_id() {
+function get_system_id() (
   local os_release
 
   os_release="$(find_os_release)"
@@ -504,8 +504,9 @@ function get_system_id() {
     return
   fi
 
-  echo $(awk -F= '/ID/ {print $2}' $os_release | head -n 1)
-}
+  . "$os_release"
+  echo "$ID"
+)
 
 
 # Returns the content of field VERSION_ID in os-release