Add automated tests for WorldEdit API functions that run under CI

uses a real Minetest instance (Docker)
currently covers most basic manipulations

.github/workflows/test.yml

@@ -0,0 +1,11 @@
+name: test
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Run tests
+      run: MINETEST_VER=latest ./util/run_tests.sh