Move all "testing-fns" APIs into libsignal-bridge-testing

This means they'll be shipped to clients, but still filtered out of
the device builds on Android (via having two separate libraries on
disk) and iOS (by not being included in non-simulator builds). The
biggest benefit of this is dropping the :android:makeTestJniLibraries
step for running libsignal's Android tests.

rust/bridge/ffi/src/lib.rs

@@ -8,7 +8,7 @@
 
 use futures_util::FutureExt;
 use libsignal_bridge::ffi::*;
-#[cfg(feature = "testing-fns")]
+#[cfg(feature = "libsignal-bridge-testing")]
 #[allow(unused_imports)]
 use libsignal_bridge_testing::*;
 use libsignal_protocol::*;