add gitlab ci

.gitlab-ci.yml

+image: gcc
+
+stages:
+  - test
+
+test:
+  stage: test
+  before_script:
+    - apt update && apt -y install make cmake
+  script:
+    - mkdir build && cd build
+    - cmake .. -DBUILD_TESTS=ON
+    - make
+    - ./validator_test