tests: new file test_helper_libs.bash

For testing non-core files, just `load ../test_helper_libs` after `load ../test_helper` instead of trying to guess which libs must be loaded, in which order, for testing to succeed.

`_command_exists` spews log messages, so our test log is littered with `_log_debug not found`. This fixes that too.

clean_files.txt

@@ -123,6 +123,7 @@ plugins/available/zoxide.plugin.bash
 #
 test/plugins/alias-completion.plugin.bats
 test/test_helper.bash
+test/test_helper_libs.bash
 
 # themes
 #