chore(gatsby-plugin-preload-fonts): don't transpile mocks

[pieh]

Description

Just minor maintaince kind of things as right now the --ignore syntax for mocks doesn't work (for me). This replicates syntax from "tests" to also be used for "mocks"

This prevents following untracked files to show after yarn bootstrap:

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        packages/gatsby-plugin-preload-fonts/__mocks__/
        packages/gatsby-plugin-preload-fonts/logger-mock.js