Build Tooling: Split JavaScript tasks to individual jobs (#15229)

* Build Tooling: Split JavaScript tasks to individual jobs

* Build Tooling: Reintroduce install script

I thought it redundant. Alas, I was wrong

* Build Tooling: Run packages custom build script in unit tests

.travis.yml

@@ -27,17 +27,40 @@ before_install:
 
 jobs:
   include:
-    - name: JS unit tests
-      env: WP_VERSION=latest
+    - name: Lint
       before_install:
         - nvm install --latest-npm
       install:
         - npm ci
       script:
-        - npm run build
         - npm run lint
+
+    - name: Build artifacts
+      before_install:
+        - nvm install --latest-npm
+      install:
+        - npm ci
+      script:
         - npm run check-local-changes
+
+    - name: License compatibility
+      before_install:
+        - nvm install --latest-npm
+      install:
+        - npm ci
+      script:
         - npm run check-licenses
+
+    - name: JavaScript unit tests
+      before_install:
+        - nvm install --latest-npm
+      install:
+        - npm ci
+      script:
+        # It's not necessary to run the full build, since Jest can interpret
+        # source files with `babel-jest`. Some packages have their own custom
+        # build tasks, however. These must be run.
+        - npx lerna run build
         - npm run test-unit -- --ci --maxWorkers=2 --cacheDirectory="$HOME/.jest-cache"
 
     - name: PHP unit tests (Docker)