rocfft-test: make accuracy test thread handling exception safe
rocfft-test: make accuracy test thread handling exception safe

If an exception was being thrown, we could throw again while unwinding the
stack because thread::~thread is called on a thread that hasn't been joined.
ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT