fix watch mode --fix with multiple files

[wolfgangwalther]

I ran into a problem, where toggling --fix via the watch plugin worked fine when I filtered the list of files to 1 file. As soon as I started watching multiple files, however, --fix did not work anymore.

It took me quite a while to figure this out, but it looks like somehow this way of creating the runner

createJestRunner(require.resolve("./run"));

leads to a situation where the import for configOverrides in run.js creates new objects for each file the runner is run on. Since this object is not the same as the one imported in the watch plugin itself, .getFix() always returned undefined, and the fix never worked.

The change in here fixes that, because configOverrides is only imported once in index.js.

This approach is actually taken from jest-runner-eslint, which worked fine for me.