Bug 578487 - added tests for RefreshJob

No real functional / behavior code changes, just refactored RefreshJob
so it can be tested and added trivial tests.

Change-Id: I7f1d339824e5b666a9239600e7da406ab6222513
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.resources/+/190369
Tested-by: Platform Bot <platform-bot@eclipse.org>
3 files changed