fix(nodejs): Add system-test/fixtures to .eslintignore

[danielbankhead]

We have some mismatches between the files eslint lints and tsconfig checks - which will cause some errors in a future release of gts and eslint.

We can prepare for this upgrade and remove this mismatch for many of our libraries, including:

• https://github.com/googleapis/nodejs-googleapis-common
• https://github.com/googleapis/google-auth-library-nodejs
• https://github.com/googleapis/gaxios
• and others with 'fixtures' in system-tests