Preparation for making repo public: Post results from demo runs as PR comments

[wenovus]

DO NOT SUBMIT. This PR is likely superseded by #13

The results from the demo runs will now be posted as PR comments. These comments are updated by subsequent pushes to the PR. Its purpose is to show the effects of the PR's set of tests on the public models. If new tests are added, we want to make sure that the tests either pass as expected, or fail as expected (if the test corrects a current model error).

Here is the expected workflow for adding a new test for fixing a bad pattern in the models:

1. Someone notices a pattern is wrong.
2. Tests are added to the openconfig/pattern-regex-tests repo, demonstrating the failure on openconfig/public.
3. PR is merged, and a new minor version of openconfig/pattern-regex-tests is released.
4. A PR is opened for openconfig/public that uses the new version of openconfig/pattern-regex-tests, with the corresponding fix to the pattern in the model.

Also updated README with the workflow steps above, and split existing regex tests into modules.

[github-actions]

XSD Demo Test Results on openconfig/public

all tests passed

[github-actions]

POSIX Demo Test Results on openconfig/public

all tests passed

[wenovus]

This PR is likely superseded by #13