ignoring return value in test

[t-hishinuma]

matrix_transpose.cpp:60:3: warning: ignoring return value of 'monolish::matrix::COO<Float>& monolish::matrix::COO<Float>::transpose() [with Float = double]', declared with attribute nodiscard [-Wunused-result]
   60 |   A.transpose();

[t-hishinuma]

matrix_common.cpp:79:5: warning: ignoring return value of 'monolish::matrix::COO<Float>& monolish::matrix::COO<Float>::transpose() [with Float = double]', declared with attribute nodiscard [-Wunused-result]
   79 |     transposed_COO1.transpose();

[t-hishinuma]

Remove the nodiscard from the transpose() function.