rganalysis: Fix compiler warnings in the unit test

rganalysis: Fix compiler warnings in the unit test

elements/rganalysis.c:919:66: error: shifting a negative signed value is undefined
      [-Werror,-Wshift-negative-value]
  push_buffer (test_buffer_const_int16_stereo (8000, 16, 512, -1 << 14, 0));
                                                              ~~ ^
elements/rganalysis.c:929:69: error: shifting a negative signed value is undefined
      [-Werror,-Wshift-negative-value]
  push_buffer (test_buffer_const_int16_stereo (8000, 16, 512, 0, -1 << 14));
                                                                 ~~ ^
elements/rganalysis.c:939:64: error: shifting a negative signed value is undefined
      [-Werror,-Wshift-negative-value]
  push_buffer (test_buffer_const_int16_mono (8000, 16, 512, -1 << 14));
                                                            ~~ ^