Dummy sensor backend can block

See the attached gdb output, it's likely that the dummy/test sensor backend is blocking unity8 startup.

(gdb) info threads
  Id Target Id Frame
  14 Thread 0xb3781410 (LWP 31081) "unity8" 0xb61e1970 in recvmsg () at ../sysdeps/unix/syscall-template.S:81
  13 Thread 0xb2f81410 (LWP 31082) "unity8" syscall () at ../sysdeps/unix/sysv/linux/arm/syscall.S:37
  12 Thread 0xb225c410 (LWP 31083) "MirServerThread" 0xb61da4e2 in poll () at ../sysdeps/unix/syscall-template.S:81
  11 Thread 0xb18a0410 (LWP 31085) "RPC Thread" 0xb61da4e2 in poll () at ../sysdeps/unix/syscall-template.S:81
  10 Thread 0xb0eff410 (LWP 31086) "arch_worker" 0xb4422840 in ?? ()
  9 Thread 0xb2523b40 (LWP 31087) "MirServerThread" __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:47
  8 Thread 0xb04ff410 (LWP 31088) "SWD-GPU Job" __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
  7 Thread 0xafcff410 (LWP 31089) "SWD-Frame" __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
  6 Thread 0xaf301410 (LWP 31090) "Input dispatch" 0xb61da4e2 in poll () at ../sysdeps/unix/syscall-template.S:81
  5 Thread 0xaeb01410 (LWP 31091) "Mir/Snapshot" __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46
  4 Thread 0xae301410 (LWP 31092) "Mir/IPC" 0xb61e1132 in epoll_wait () at ../sysdeps/unix/syscall-template.S:81
  3 Thread 0xadb01410 (LWP 31093) "Mir/IPC" 0xb61e1132 in epoll_wait () at ../sysdeps/unix/syscall-template.S:81
  2 Thread 0xac76d410 (LWP 31094) "unity8" 0xb61d6ae0 in read () at ../sysdeps/unix/syscall-template.S:81
* 1 Thread 0xb6f9f000 (LWP 31079) "unity8" __libc_do_syscall () at ../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:46

Thread 2 (Thread 0xac76d410 (LWP 31094)):
#0 0xffffffff in read () at ../sysdeps/unix/syscall-template.S:81
#1 0xffffffff in SensorController::fifo_take_command() (__nbytes=1, __buf=0xac76cdcf, __fd=<optimized out>) at /usr/include/arm-linux-gnueabihf/bits/unistd.h:44
#2 0xffffffff in SensorController::fifo_take_command() (this=0xac7861e8 <SensorController::instance()::_inst>) at /build/platform-api-1_CRAF/platform-api-3.0.1+15.04.20151127/src/ubuntu/application/testbackend/ubuntu_application_sensors.cpp:287
#3 0xffffffff in std::thread::_Impl<std::_Bind_simple<SensorController::SensorController()::<lambda()>()> >::_M_run(void) (__closure=0xd46f68)
    at /build/platform-api-1_CRAF/platform-api-3.0.1+15.04.20151127/src/ubuntu/application/testbackend/ubuntu_application_sensors.cpp:233
#4 0xffffffff in std::thread::_Impl<std::_Bind_simple<SensorController::SensorController()::<lambda()>()> >::_M_run(void) (this=0xd46f68) at /usr/include/c++/4.9/functional:1700
#5 0xffffffff in std::thread::_Impl<std::_Bind_simple<SensorController::SensorController()::<lambda()>()> >::_M_run(void) (this=0xd46f68) at /usr/include/c++/4.9/functional:1688
#6 0xffffffff in std::thread::_Impl<std::_Bind_simple<SensorController::SensorController()::<lambda()>()> >::_M_run(void) (this=0xd46f5c) at /usr/include/c++/4.9/thread:115
#7 0xffffffff in std::(anonymous namespace)::execute_native_thread_routine(void*) (__p=<optimized out>) at ../../../../../src/libstdc++-v3/src/c++11/thread.cc:84
#8 0xffffffff in start_thread (arg=0x0) at pthread_create.c:335
#9 0xffffffff in () at ../sysdeps/unix/sysv/linux/arm/clone.S:89

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: ubuntu-application-api3-test-dbgsym 3.0.1+15.04.20151127-0ubuntu1 [origin: LP-PPA-ci-train-ppa-service-landing-022]
Uname: Linux 3.4.67 armv7l
ApportVersion: 2.17.2-0ubuntu1.3touch1
Architecture: armhf
Date: Tue Dec 8 15:08:53 2015
InstallationDate: Installed on 2015-12-08 (0 days ago)
InstallationMedia: Ubuntu 15.04 - armhf (20151208-020303)
SourcePackage: platform-api
UpgradeStatus: No upgrade log present (probably fresh install)