test/system: Be more strict when checking the version

https://github.com/containers/toolbox/pull/1165

test/system/001-version.bats

@@ -9,7 +9,10 @@ setup() {
 }
 
 @test "version: Check version using option --version" {
-  run $TOOLBOX --version
+  run --separate-stderr $TOOLBOX --version
 
+  assert_success
   assert_output --regexp '^toolbox version [0-9]+\.[0-9]+\.[0-9]+(\.[0-9]+)?$'
+  assert [ ${#lines[@]} -eq 1 ]
+  assert [ ${#stderr_lines[@]} -eq 0 ]
 }