feat(cmake): add option for disabling tests

[GunniBusch]

This pull request updates the project's CMakeLists.txt to improve how testing is enabled and managed. The main changes introduce a new option for controlling test builds, conditionally include test dependencies and directories, and improve reproducibility by specifying a hash for the googletest dependency.

Testing configuration improvements:

• Added a new CMake option NBYTES_ENABLE_TESTING to control whether testing is enabled, defaulting to the value of BUILD_TESTING (option(NBYTES_ENABLE_TESTING "Enable testing" ${BUILD_TESTING})).
• Now only adds the tests subdirectory and fetches googletest if testing is enabled, making test dependencies optional for non-testing builds.
• Added a SHA256 hash for the googletest dependency to ensure consistent and secure downloads.

General build system improvements:

• Moved the addition of the src subdirectory to occur before the conditional test logic, ensuring core sources are always included.

PS: Only the pr description was generated via ai.
PPS: Tests are on by default and can be controlled via BUILD_TESTING or NBYTES_ENABLE_TESTING.