Proper CONCAT, CONCAT_DISTINCT support for parent multi-select fields

• Fixes #53 by ensuring string-based lookups conform to the multi-select picklist shape for concat in particular
• Fixed an issue with the build script that didn't always report test failures (most likely due to a breaking change in the CLI, as the build used to bomb out on test failure). We now explicitly check the test result status in case the exit code for the test command isn't returned to the shell correctly