Sample-tester

sample-tester: Prettify test error messages
gen-manifest: ignore YAML files; correctly quote values in the manifest.

This release fixes the broken `--cases` flag and allows logging to stderr to work if requested.

* Allow user to pass an arbitrary number of glob expressions in the `sample-tester` command line. These globs should resolve to YAML files containing configs (testplans or manifests), which now self-identify by a top-level `type` field.
* Switch to `click` for arg parsing in `gen-manifest` in order to allow gen-manifest to process CLI-provided tags with spaces
* Fix assorted errors: typos, bugs, flake8 warnings

Just the one minor usability improvement

For backward compatibility, 'assert_not_contains' is now a (deprecated) alias for 'assert_excludes'