Add unit test on MacOS using Github Actions (#14220)

- Remove macos tests from travis
- Add macos tests in github actions.

.travis.yml

@@ -76,10 +76,6 @@ jobs:
       os: linux
       language: python
       env: TEST_SUITE=doc
-    - os: osx
-      language: generic
-      env: [ TEST_SUITE=unit, PYTHON_VERSION=2.7, COVERAGE=true ]
-      if: type != pull_request
 
 stages:
   - 'style checks'
@@ -111,15 +107,6 @@ addons:
       - r-base-core
       - r-base-dev
       - zsh
-  # for Mac builds, we use Homebrew
-  homebrew:
-    packages:
-      - python@2
-      - gcc
-      - gnupg2
-      - ccache
-      - dash
-      - kcov
     update: true
 
 # ~/.ccache needs to be cached directly as Travis is not taking care of it
@@ -130,14 +117,7 @@ cache:
   directories:
     - ~/.ccache
 
-# Work around Travis's lack of support for Python on OSX
 before_install:
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
-        pip2 install --upgrade pip;
-        pip2 install virtualenv;
-        virtualenv venv;
-        source venv/bin/activate;
-    fi
   - ccache -M 2G && ccache -z
 
 # Install various dependencies