fix(spanner): resolve prelint, eslint, and prettier CI build failures

[sakthivelmanii]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds 'system-test/fixtures' to the ESLint ignore list, removes the 'prelint' script from 'package.json', and adds a trailing comma to an assertion in 'system-test/spanner.ts'. I have no feedback to provide as there are no review comments.