test/system: Rewrite a test using the standard Bats helpers

The test was earlier rewritten in commit b0beb68 with custom
code in the hope that it would make it more reliable.  The test has
proven to be reliable in recent times, and the cause for its past
unreliability is unclear.  Therefore, it will be better to remove the
custom code in favour of the standard Bats helpers for the sake of
consistency and readability.

Until Bats 1.10.0, 'run --keep-empty-lines' had a bug where it counted
the trailing newline on the last line as a separate line [1].  However,
Bats 1.10.0 is only available in Fedora >= 39 and is absent from Fedora
38.

[1] Bats commit 6648e2143bffb933
    bats-core/bats-core@6648e2143bffb933
    bats-core/bats-core#708

containers#1506

test/system/104-run.bats

@@ -203,16 +203,18 @@ teardown() {
 @test "run: 'sudo id' inside the default container" {
   create_default_container
 
-  output="$("$TOOLBX" run sudo id 2>"$BATS_TEST_TMPDIR/stderr")"
-  status="$?"
-
-  echo "# stderr"
-  cat "$BATS_TEST_TMPDIR/stderr"
-  echo "# stdout"
-  echo "$output"
+  run --keep-empty-lines --separate-stderr "$TOOLBX" run sudo id
 
   assert_success
-  assert_output --partial "uid=0(root)"
+  assert_line --index 0 "uid=0(root) gid=0(root) groups=0(root)"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  assert [ ${#stderr_lines[@]} -eq 0 ]
 }
 
 @test "run: Ensure that /run/.containerenv exists" {