Get rid of global state in carbonapi app tests

[grzkv]

Currently, the key tests in carbonapi depend on global variables https://github.com/bookingcom/carbonapi/blob/b29e91ecfe7f8ab1c78a520a1d9fd4e0da88df68/app/carbonapi/app_test.go#L21

This makes the test results depend on test order and requires implicit operations. The global state needs to be removed and replaced with a local one.