tests: introduce catch2 unit tests infrastructure.

tests: introduce catch2 unit tests infrastructure.

Catch2 is a multi-paradigm test framework for C++.
It is primarily distributed as a single header file,
very easy and simple to adopt this to thorvg project.

This patch introduces catch2 infrastsructure and one prototype as a sample.

You can refer "testInitializer.cpp", how to add unit test!
while ignoring else files such as "catch2.hpp", "testMain.cpp"

Also, enable Unit-tests with meson option when you change any thorvg code.

$meson build -Dtests=true.

launch tvgUnitTest in the build result then verify 100% coverage
before submitting any patches.

Change-Id: Icc09d4402dc38c8a92a1cebb2cb502728c092f36