Fix serving test data on windows

[M393]

On windows the path was also prefixed with a slash like /C:\\geotiff.js\\test and then it tried to serve files from C:\\C:\\geotiff.js\\test

[constantinius]

@M393 I fixed the missing file issue. Could you please update from master and try again?

[M393]

This issue is not related to the missing test image. No files at all were served on windows.

• import.meta.url is e. g. 'file:///C:/geotiff.js/test/geotiff.spec.js'
• new URL(import.meta.url).pathname becomes '/C:/geotiff.js/test/geotiff.spec.js',
  note the additional slash at the start,
• serve-static uses resolve from the path module, which finally results in 'C:\\C:\\geotiff.js\\test'.

fileURLToPath is the platform-independent way of getting the file path.