Util::matrixMatrixProduct produces a wrong result with BLAS

[chaeyeunpark]

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Issue description

Description of the issue - include code snippets and screenshots here
if relevant. You may use the following template below

• Expected behavior: (What you expect to happen)
  C++ tests should pass with BLAS

• Actual behavior: (What actually happens)
  Tests fail

• Reproduces how often: (What percentage of the time does it reproduce?)

• System information: (post the output of import pennylane as qml; qml.about())

This is in fact caused by two convoluted problems:

1. matrixMatProd(m_left, m_right, .., transpose=true) function in Util.hpp transpose m_right (expected behaviour) when BLAS is not used, but BLAS version transpose m_left.
2. Currently C++ tests do not use BLAS even when -DENABLE_BLAS=ON is given as tests/CMakeLists.txt does not include external library or compile flags set by the options.

Fortunately, as wheels are built with DENABLE_BLAS=OFF, this does not cause an issue for usual users (unless someone compiles from the source directly). Still, I will fix this ASAP.