* Manual Xcode project fixups
* Remove `firebase/firestore` from paths in Firestore/core
This renames all Firestore files under `Firestore/core` to remove the
interior `firebase/firestore` directory structure. This is motivated by
build failures under Windows where the filenames of intermediates were
too long.
Concretely:
* `Firebase/core/src/firebase/firestore` becomes `Firebase/core/src`
* `Firebase/core/test/firebase/firestore` becomes
`Firebase/core/test/unit`
Moving the tests into a `unit` subdirectory aligns Firestore with the
directory structure in
[AddNewPod](https://github.com/firebase/firebase-ios-sdk/blob/master/AddNewPod.md),
modified for C++ (see discussion in #3224).
* Fix unused variable warnings
These are now exposed because libprotobuf is no longer polluting the
compiler command-line with -Wno-unused-parameter.
* Convert (void)var to __unused where possible
* Add FIRFirestoreTests to the Firestore Xcode project
* Avoid waitForExpectations:timeout:
This API was added in Xcode 8.3, but we still build production releases
with Xcode 8.2. waitForExpectationsWithTimeout:handler: is available
from Xcode 7.2.
* Add AppForUnitTesting
Add a utility for constructing a Firebase App for testing.
* Handle the nil UID from FIRAuth
* Avoid running CMake tests twice
* Only build app_testing on Apple platforms
* Revise test.sh messages
Paul Beusterien