Exclude testGetters and golemrunner from non-standalone builds

Marco De Luciarequested to merge
exclude-test-executables into main

Move testGetters and golemrunner executables inside the BUILD_TESTING and STANDALONE_BUILD conditional block. This prevents these test executables from being built when litephreeqc is used as a submodule, as in the revamped PhreeqConnector R package and the new vibe-coded pyPhreeqConnector binding for python.

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