Discover SIO when finding and running tests explicitly

[jmcarcell]

BEGINRELEASENOTES

• Discover SIO when finding and running tests explicitly. This is necessary if SIO is not in LD_LIBRARY_PATH, for example, when it is built in.

ENDRELEASENOTES

I'm not sure if no one does this (I think this used to work fine?) but if I build LCIO with builtin SIO then SIO is not in LD_LIBRARY_PATH, so I get:

CMake Error at /usr/lib/cmake/Catch2/CatchAddTests.cmake:123 (message):
  Error listing tests from executable
  '/home/juanmi/Key4hep/LCSoft/LCIO/build/bin/unittests_lcio':

    Result: 127
    Output:

Call Stack (most recent call first):
  /usr/lib/cmake/Catch2/CatchAddTests.cmake:280 (catch_discover_tests_impl)

It makes sense; the local build of SIO is not in LD_LIBRARY_PATH, and even if RPATHS are set and correct (I checked) this is Catch2's discovery (DL_PATHS I'm not sure exactly what it does). The change in LD_LIBRARY_PATH is probably (didn't check) needed when rpaths are not enabled and the SIO library is not in LD_LIBRARY_PATH.