oci_discovery/uri_template/test: Test URITemplate.__str__

[wking]

This gives us complete coverage for the uri_template package, as measured by:

$ pip install --user coverage
$ python -m coverage run -m unittest discover
$ python -m coverage report | grep uri_template
oci_discovery/uri_template/__init__.py        10      0   100%
oci_discovery/uri_template/test.py            33      5    85%

Incomplete coverage in test.py is acceptable; the unexercised statements are due to:

• The presence of uritemplate on my local system, so I don't excercise the ImportError fallback. The fallback allows the tests to run on systems without uritemplate, so we want to keep it.

• The lack of buggy double entries, so I don't exercise the “entries in both” failure. This guard protects devs from adding buggy double entries, so we want to keep it.

• The absence of any unexpected failures, so I don't exercise the raise. We want to keep it to get good messages if/when an implementation does a worse job of implementing the RFC.

• The presence of all expected failures, so I don't exercise the “no longer raises an exception” failure. We want to keep it to get good messages if/when an implementation does a better job of implementing the RFC.

[xiekeyang]

LGTM