Fix for test error with Intel compiler

[G-Ragghianti]

With the 2024 version of the oneapi compilers, the following error occurs:

/tmp/blaspp/test/test_util.cc:566:45: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
  566 |         blas::Queue::stream_t* stream_ptrs[ MAX_DEVICES * repeat ];
      |                                             ^~~~~~~~~~~~~~~~~~~~
/tmp/blaspp/test/test_util.cc:566:59: note: read of non-const variable 'repeat' is not allowed in a constant expression
  566 |         blas::Queue::stream_t* stream_ptrs[ MAX_DEVICES * repeat ];
      |                                                           ^
/tmp/blaspp/test/test_util.cc:400:9: note: declared here
  400 |     int repeat = 4;
      |         ^

This fix makes 'repeat' a const.

[mgates3]

This is already fixed in #91.