feat: auto-detect test executables via CMake Tools

[haikion]

Adds automatic detection of Google Test executables via CMake Tools. This is largely based on the discussions in issue #12. The submitted patch sort of worked but required a few tweaks. Permission to use the patch was granted, and credit will be given to the original authors in the form of a code comment. I also added a couple of tests. Feature has some imperfections, which I consider out of scope.

Working

• Automatic test executable discovery for a file
• Automatic test executable discovery for a test fixture
• Automatic test executable discovery for a test

Known issues

• Does not automatically discover executables for a directory
• Issues may arise if the current working directory is not in the project root

I omitted implementation for directory-based discovery, as neotest-gtest seems to assume that a directory maps to a single test executable, which is not always the case. cmake-tools expects the current working directory to be in the project root, so fixing such issues would quickly snowball.

closes #12