macOS rpath is not defined correctly

[c-dilks]

MacOS tests from @tongtongcao indicate there may be problems with example executable rpaths. To maintain stability on MacOS and to see if we can reproduce such rpath issues, let's add MacOS tests to the CI.

[c-dilks]

#73 reproduces:

dyld[4753]: Library not loaded: '@rpath/libhipo4.dylib'
  Referenced from: '/Users/runner/work/iguana/iguana/iguana/bin/iguana-example-00-basic'
  Reason: tried: '$ORIGIN/../lib:/Users/runner/work/iguana/iguana/hipo/lib/pkgconfig/../../lib/libhipo4.dylib' (no such file), '$ORIGIN/../lib:/Users/runner/work/iguana/iguana/hipo/lib/pkgconfig/../../lib/libhipo4.dylib' (no such file), '/usr/local/lib/libhipo4.dylib' (no such file), '/usr/lib/libhipo4.dylib' (no such file)