fix(ConnectionMock.spec.js): monkey-patch with PromiseMock for these tests

Previously, tests were failing due to depending half on patched code, and half
on native promises which did not interact with the mocks as expected. Calling
PromiseBackend.patchWithMock() before each test, and restoring the native
promise after each test, corrects this behaviour. Ideally, this should occur
for all test files, but for now the problem is limited to
ConnectionMock.spec.js.

Closes #18