Test multiple globals with the same name

Is it currently possible to use one Karma config to test all implementations of some global?

Use case: test multiple strategies for some shim.

Found no easy way to do the following:

``` javascript
// src/a.js
function foo() { return 'a' }
// test/aSpec.js
expect(foo()).toBe('a');

// src/b.js
function foo() { return 'b' }
// test/bSpec.js
expect(foo()).toBe('b');
```

Start by reviewing Karma's configuration and file-loading behavior for the example src/a.js, src/b.js, test/aSpec.js, and test/bSpec.js files. Determine whether one run can isolate implementations that define the same global, and identify the relevant tests for configuration or browser context setup. Done means documenting or implementing a supported way to test both strategies without global-name collisions.

Written by the indexing model from the issue text.