Fix Eslint issues found in block-serialization-spec-parser packages

[gziolo]

Description

Fixes issue raised by @aduth in the comment:

Why do we ignore the entire package? Could we just ignore the singular file(s) which are generated? The test file, for example, includes a number of legitimate code style violations.

It turned out that using toMatchInlineSnapshot doesn't work quite well for use because it uses prettier behind the scenes. We can't really use it with the current setup unless we adopt calypso-prettier.

This PR ensures that only generated file is ignored by lint checks and updates existing test suite code to pass Eslint checks.

How has this been tested?

npm test

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[aduth]

👍 Thanks.