fix: ECR gate representation and more correctness tests

[kshyatt-aws]

Issue #, if available: N/A

Description of changes: ECR gate matrix representation was skewed which didn't affect probabilities but did affect amplitude phases. @speller26's correctness tests revealed this which is now fixed, and the other custom gate correctness tests are added.

Testing done: Unit tests passed locally.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• [x] I have read the CONTRIBUTING doc
• [x] I used the commit message format described in CONTRIBUTING
• [x] I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• [x] I have added tests that prove my fix is effective or that my feature works (if appropriate)
• [x] I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

The author of this PR, kshyatt-aws, is not an activated member of this organization on Codecov. Please activate this user on Codecov to display this PR comment. Coverage data is still being uploaded to Codecov.io for purposes of overall coverage calculations. Please don't hesitate to email us at support@codecov.io with any questions.