Testing Bitcoin Core source code?
What's the recommended way to start testing the Bitcoin Core source code?
testing for verification of bug fixes

See https://github.com/bitcoin/bitcoin/blob/master/src/test/README.md which says:

To run the bitcoind tests manually, launch src/test/test_bitcoin. To recompile after a test file was modified, run make and then run the test again. If you modify a non-test file, use make -C src/test to recompile only what's needed to run the bitcoind tests.
To add more bitcoind tests, add BOOST_AUTO_TEST_CASE functions to the existing .cpp files in the test/ directory or add new .cpp files that implement new BOOST_AUTO_TEST_SUITE sections.
To run the bitcoin-qt tests manually, launch src/qt/test/test_bitcoin-qt
To add more bitcoin-qt tests, add them to the src/qt/test/ directory and the src/qt/test/test_main.cpp file.

There is more.