C++: Autoload schema catalogues and set up environment for tests

More low-level infrastructural bits:

- Autoload the schema catalogues in libome-xml.  This uses the schema path introspection added in earlier PRs; i.e. use BIOFORMATS_SCHEMADIR or BIOFORMATS_HOME to override
- When running tests, set BIOFORMATS_SCHEMADIR
- When running by hand, use the bf (sh) or bf.bat (cmd) wrapper to set up the environment to find things in the build tree; running directly will now fail since it depends upon the schema data

While the above changes are initially just for the schema data, the same infrastructure will later be used to find other data, libraries, programs, loadable modules, configuration, etc..

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Testing:

- The tests should remain green if the test environment is correct
- Try running `cpp/test/ome-bioformats/version` in the build tree; it should fail (abort with exception)
- Try running `./bf cpp/test/ome-bioformats/version` in the build tree; it should pass