Add upload-node maze runner tests

[imjoehaines]

Goal

This PR adds tests for the upload-node subcommand. These are mostly the same as the browser tests, because there aren't really an specific Node bundlers e.g. Next.js uses Webpack under the hood, but I did add a fixture that uses Babel's CLI to compile the same code against node & browser targets

Changeset

• Split Buildkite pipeline into browser & node steps
• Add source-map-babel fixture; this has 4 possible build options against old versions (so it does a lot of transformations):
  • single sourcemap node
  • multiple sourcemap node
  • single sourcemap browser
  • multiple sourcemap browser
• Add copies of the upload-browser tests that run upload-node (main difference is node doesn't need a URL)
• Add tests against the new source-map-babel fixture for node & browser

[bengourley]

One note about minifiedUrl for Node to address.

Another thing it would be good to test at this level is missing sourcesContent from the source map. This applies to both Node and Browser.

If you configure TypeScript, Babel or Webpack to not include that in the source map that is created (e.g. with typescript this can be switched off with the inlineSources compiler option, or with webpack devtool: 'nosources-source-map') and then assert that the received source map contains sourcesContent that should suffice.

The sourcesContent stuff can be done as a follow-on and doesn't have to hold up this PR.