Running jest with multiple configs

[aaronabramov]

add the ability to run Jest with multiple configs.

Example:
jest --config ios.json --config android.json

the idea is that we run all tests for the configs within a single process and then aggregate the results for all of them in the reporters.

some of the challenges:

• what do we do with global configurations like testCoverageDir, verbose, or watch?
• how do we watch files for both configs?

@cpojer @kentaromiura

[cpojer]

I think there are two things here that are kind of conflated:

• Have a single test run with N configs. These are the same global configs we have currently, they have their own haste-map instances and we'd change all APIs (like watch mode) to take an array of haste maps instead of just one. If we don't do this, killing our internal runners will be much harder or we'll need to rewrite it anyway and open source it eventually. But basically, this is the incremental path to get rid of the shitty internal code we have.
• Have a single test run with 1 config but enable a system of local-configs that can adjust how tests behave. One such example is having nested .jestrc files, another one would be @directive directives. This requires to split up our configuration system into a global one and defining a subset of the global configuration that can act as a local option (for example: testEnvironment). None of these should be able to amend the selection of JS files, except maybe to ignore/find certain test files (would need some modifications to track this in the haste map but I think it is possible).

Shall we split up these two into separate issues? Which one should we go deeper into?

Also cc @pedrottimark.

[cpojer]

Duplicate of #2970

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