[Extensions] Reduce boilerplate for external extension tests

Two changes collaborate here to reduce the boilerplate:

- Make a specialized static function to store the external extension
  path for tests, so no reference to a concrete XWalkExtensionService
  is needed.

- Now that we do not need the reference use SetUp() function instead of
  the callback for registering extensions in tests. This change make the
  service passed in that callback unused, we'll remove in the next patch
  to keep this easier to review.

A helper function was added to get the path where an external extension
built for test is.
[Extensions] Remove service parameter from register test callback

XWalkExtensionService allows us to register a function to be called
every time a new XWalkExtensionServer is created, so tests can register
specific extensions. This function may be called multiple times because
we create a (in process) server per Render Process created.

The callback also received a service parameter, to be used for setting
the path for external extensions to be loaded. But the previous commit
changed how tests handle this, so the parameter can be removed.
[Extensions] Add RegisterExtensionForTest() function

This is a small convenience function that reduces the boilerplate when
registering extensions in tests.
Merge pull request #1101 from cmarcelo/reduce-extension-tests-boilerplate

[Extensions] Reduce extension tests boilerplate