message: ci: Document why tests can not be run on mac
diff: @@ -10,6 +10,7 @@ export HOST=x86_64-apple-darwin16
  export PIP_PACKAGES="zmq"
  export GOAL="install"
  export BITCOIN_CONFIG="--enable-gui --enable-reduce-exports --enable-werror"
  +export TEST_RUNNER_EXTRA="wallet_disable" # Only run wallet_disable as a smoke test, see https://github.com/bitcoin/bitcoin/pull/17240#issuecomment-546022121 why the other tests are disabled
  # Run without depends
  export NO_DEPENDS=1
  export OSX_SDK=""
  