Add a GitHub workflow for testing

Signed-off-by: Johannes Schindelin <[email protected]>

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+name: Test Posh-Git
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: install dependencies
+        shell: pwsh
+        run: |
+          "Git version: $(git --version)"
+          "PSVersion:   $($PSVersionTable.PSVersion)"
+          "Host name:   $($Host.Name)"
+
+          Set-PSRepository -Name PSGallery -InstallationPolicy Trusted
+          Install-Module Pester -MinimumVersion 5.0.0 -MaximumVersion 5.99.99 -Scope CurrentUser -Force
+      - name: run tests
+        shell: pwsh
+        run: |
+          Import-Module Pester -PassThru
+          $ErrorActionPreference = 'Continue'
+          $res = Invoke-Pester -Path test -Output Detailed -PassThru -ErrorAction SilentlyContinue
+          if (!$res -or ($res.FailedCount -gt 0)) {
+            $Error | Format-List * -Force
+            exit 1
+          }