build: add a `Prepare ccache` job in Travis

Combined compile and test of Node.js where lots of files need to be
compiled (e.g. after a V8 update) is exceeding the time limit for
Travis jobs (50 minutes).

Add a job to Travis that compiles Node.js but doesnt run any tests to
populate the ccache. Introduce staging and move the `Test Suite` job
into a later stage so that it can use the populated ccache.

PR-URL: https://github.com/nodejs/node/pull/27002
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.travis.yml

@@ -2,9 +2,13 @@ language: cpp
 cache: ccache
 os: linux
 dist: xenial
+stages:
+  - check
+  - test
 matrix:
   include:
     - name: "First commit message adheres to guidelines at <a href=\"https://goo.gl/p2fr5Q\">https://goo.gl/p2fr5Q</a>"
+      stage: check
       if: type = pull_request
       language: node_js
       node_js: "node"
@@ -13,13 +17,27 @@ matrix:
             bash -x tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST};
           fi
     - name: "Linter"
+      stage: check
       language: node_js
       node_js: "node"
-      env:
-        - NODE=$(which node)
       script:
-        - make lint
+        - NODE=$(which node) make lint
+    - name: "Prepare ccache"
+      stage: check
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6
+      install:
+        - export CC='ccache gcc-6' CXX='ccache g++-6' JOBS=2
+        - ./configure
+        - make -j2 V=
+      script:
+        - true
     - name: "Test Suite"
+      stage: test
       addons:
         apt:
           sources: