Rollup merge of #106689 - GuillaumeGomez:rustdoc-gui-files-array, r=notriddle

Fix invalid files array re-creation in rustdoc-gui tester

It fixes the error <code>expected `runTest` first argument to be a string</code>:

```
{
  file_name: {
    file_name: '/home/imperio/rust/rust/src/test/rustdoc-gui/type-declation-overflow.goml',
    output: 'type-declation-overflow... FAILED\n' +
      '...'
  },
  output: Error: expected `runTest` first argument to be a string
      at runTest (/home/imperio/rust/rust/node_modules/browser-ui-test/src/index.js:591:15)
      at runTests (/home/imperio/rust/rust/src/tools/rustdoc-gui/tester.js:144:26)
      at main (/home/imperio/rust/rust/src/tools/rustdoc-gui/tester.js:278:15)
} Error: expected `runTest` first argument to be a string
    at runTest (/home/imperio/rust/rust/node_modules/browser-ui-test/src/index.js:591:15)
    at runTests (/home/imperio/rust/rust/src/tools/rustdoc-gui/tester.js:144:26)
    at main (/home/imperio/rust/rust/src/tools/rustdoc-gui/tester.js:278:15)
```

The problem was that I concatenated two arrays of object whereas `files` is supposed to be an array of string.

r? `@notriddle`