[Suggestion] Change to doctest for testing

[tusharpm]

The PR is to facilitate the discussion. Please comment if the changes need to be in smaller chunks or not required.

Doctest is a single-header testing framework for C++. This section describes the differences from googletest.

Change description:

• Simplified setup with CMake. No hacks required for CRT linkage.
• Update test framework macros:
  • s/EXPECT_TRUE/CHECK/
  • s/EXPECT_/CHECK_/
  • s/TEST/TEST_CASE/ (also: remove first parameter, quote second parameter).
  • DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN to use main provided by doctest.
• Minor code updates in the tests:
  • remove compare parameter for sort as they are implicitly available.
  • fix scope for loop iterators