check the tests folders with eslint not just src, fix existing errors

[DustinAlandzes]

This pull requests adds the test folder to be checked by eslint

commandByte and metadata are unused variables

and this line gave me the "strict-booleans/no-nullable-numbers" error
https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/strict-boolean-expressions.md

const lastFrame = metadata.lastFrame || game.getLatestFrame().frame;

101:23 error Unexpected nullable number value in conditional. Please handle the nullish/zero/NaN cases explicitly strict-booleans/no-nullable-numbers

[vinceau]

Superseded by 0e0c3a2