Remove redundant assertions on `uri`

The `path` property addresses the variable which for the test. The `uri`
merely prepends the scheme and any additional origin-level attributes.
Add delimited tests for each node type
Support specifying explicit tests when running the test_suite.py

The arguments can be paths to modules (containing test cases), specific
test cases, or specific methods on a test case.

Note that bash expansion will work as exepcted, e.g. tests.backends.{sqlite,excel}
Add Excel tests for node types