Fix memory overflow in sparse matrix stress test

[anurudhp]

Reduces the input size for test_matrix_stress so that the tensors can be computed.

For the old parameters, the nightly fails with a memory error:

FAILED qualtran/bloqs/block_encoding/sparse_matrix_test.py::test_matrix_stress - numpy.core._exceptions._ArrayMemoryError: Unable to allocate 64.0 GiB for an array with shape (16, 131072, 2048) and data type complex128

[anurudhp]

@mpharrigan ptal

[mpharrigan]

thanks!