Create Github Action to run tests (#8569)

The published workflow tests the installer and checks the syntax of known zsh files
in the project.
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

.github/workflows/check-suite.yml

+name: Check Suite
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+    branches:
+      - master
+  push:
+    branches: 
+      - master
+
+jobs:
+  tests:
+    name: Run tests
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    steps:
+      - name: Set up git repository
+        uses: actions/checkout@v2
+      - name: Install zsh
+        if: runner.os == 'Linux'
+        run: sudo apt-get update; sudo apt-get install zsh
+      - name: Test installer
+        run: sh ./tools/install.sh
+      - name: Check syntax
+        run: |
+          for file in ./oh-my-zsh.sh \
+                      ./lib/*.zsh \
+                      ./plugins/*/*.plugin.zsh \
+                      ./plugins/*/_* \
+                      ./themes/*.zsh-theme; do
+            zsh -n "$file" || return 1
+          done

README.md

@@ -14,6 +14,7 @@ Finally, you'll begin to get the sort of attention that you have always felt you
 
 To learn more, visit [ohmyz.sh](https://ohmyz.sh), follow [@ohmyzsh](https://twitter.com/ohmyzsh) on Twitter, and/or join us on Discord.
 
+![Check Suite](https://github.com/ohmyzsh/ohmyzsh/workflows/Check%20Suite/badge.svg)
 [![Follow @ohmyzsh](https://img.shields.io/twitter/follow/ohmyzsh?label=Follow+@ohmyzsh&style=flat)](https://twitter.com/intent/follow?screen_name=ohmyzsh)
 [![Discord server](https://img.shields.io/discord/642496866407284746)](https://discord.gg/bpXWhnN)