Fix up more tests, keep other tests on main context

- Change context creation in ContextManager to avoid using dispatch_once, as it prevents recreation. ‘check and create’ should be the equivalent as before.

- Add a choice of context to WPAccount removeDefaultWPAccount with the default being the background context. The test should use the main context.

- Known issues: account tests cause exceptions due to a performBlock: save from another test in conjunction with the persistent store recreation. Cocoa error 134030.