[GH #887] Set darwin DYLD_LIBRARY_PATH when testing
[GH #887] Set darwin DYLD_LIBRARY_PATH when testing

Set DYLD_LIBRARY_PATH in run_command() as this allows testing from
the command-line also.

Also fix Parrot::Test::path_to_parrot, which failed when INC was
just lib/Parrot/Config.pm. Use the more reliable __FILE__ instead
and check if short, relative path seperately.

BTW: This method should be thrown away, as it does not work on an
installed parrot.

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