Fix the tests build

[vkutuev]

Short description

Building project using build.py failed with

tests/test_opencl_merge.cpp:470:14: error: ‘sort’ is not a member of ‘std’
tests/test_vector.cpp:430:14: error: ‘sort’ is not a member of ‘std’

Also, there are many warnings with comparison of integers of different signs in

List of changes

• Added inclusion of the algorithm header in the corresponding files
• Change index type in cycles in tests/test_vector.cpp
• Fix imports in tests/test_opencl_merge.cpp

Type of changes

• bug-fix (change which fixes some issue)
• new-feature (change which proposes new feature or functionality)
• breaking-change (change which brakes compatibility of api or etc.)

Changes aspect

• unit-tests for C/C++ code added
• benchmark for C/C++ code added
• workflow tests to .github scripts added
• C bindings to C++ code added
• Python bindings to C code added
• documentation to C/C++ code added
• documentation to Python code added