Merge #18799: ci: Document why tests can not be run on mac

fa72a75 ci: Document why tests can not be run on mac (MarcoFalke)

Pull request description:

  Fixes #18794

Top commit has no ACKs.

Tree-SHA512: 297652eda412aa8cf7255e20a6f294d22773dad8637a3d7b5204f3b638e911ce5b2e40e85f81395a34c1b5a5b497665944c2d6ea17c70c30c0c9e0ab553f956e

Author: MarcoFalke

ci/test/00_setup_env_mac_host.sh

@@ -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=""